Kibble is a feature of many Transformers, both in their toys and in their animation/comic designs. In its modern usage (see below for earlier meanings) it refers to pieces which have no clear purpose in one mode but are there only because they are part of another mode. In most cases, "kibble" is used to refer to pieces of the character's altmode which "hang" or stick out in their robot mode. Sometimes these pieces are there as a matter of aesthetics, and sometimes it's simply because there wasn't any place better to put the stuff.
Much less frequently, the word is used to refer to "robot parts" which are badly hidden in an altmode; This is most common in figures with aircraft altmodes, where whole limbs or compressed torsos were left exposed on the underside. In this case, the term undercarriage junk is more often used. During the Beast Wars era, the term was also applied to any figure with robot mode limbs hanging from underneath the animal-mode shell.
Parts of one mode which are merely visible in other modes are not necessarily kibble. For example, an altmode part which is well-integrated into the robot's form -- like the front of a car or truck that becomes a character's chest -- is not kibble because it is an integral part of the robot's body, rather than being "tacked on". Similarly, in the original form of Beast Wars Megatron, the T-Rex head for a hand is not kibble, as he uses it both as a laser and as a vice grip.
When "kibble" entered the Transfan lexicon, it had a different meaning, and referred to a Transformer's accessories and detachable parts which had nowhere to go when not attached/worn/held. This meaning has almost, if not completely, disappeared from use.
Examples of kibble
Although it was most prominent during the Beast Era and Robots in Disguise, kibble has been a part of Transformers from the very beginning. G1 Prowl, for example, has car doors that stick out behind his shoulders. The doors have no purpose in robot mode. They are not part of his actual body; they are just there, serving as decoration, but otherwise pointless. Those doors are kibble. In the robot modes of the G1 Seeker toys, the jet's nosecone and tailfins are also kibble. The jet wings, despite being one of the Seekers' most prominent features, are kibble as well because they are not needed for the robot mode. In the old sense of the word, their fists can also be considered kibble since they are removed for jet mode yet do not store anywhere in said mode.
One of the most infamous examples of beast kibble on a robot mode is Big Convoy from Beast Wars Neo. Many of the Neo toys are kibble heavy, with "panels" of beast skin attached to the robot body by swiveling rods. This is also common in the Car Robots/Robots in Disguise line, most notably with the Autobot Brothers (Prowl, Side Burn, and X-Brawn).
Kibble is usually only brought up when it is being used derisively. Indeed, kibble often has a negative impact on a toy. It can restrict movement by getting in the way, it can make a toy unbalanced if it is too concentrated in one part of the body such as a "backpack", and it can simply make the robot look like a purely humanoid robot that has car or animal parts stuck to it, rather than being a robot composed of car or animal parts.
However, kibble serves an important purpose in Transformer design, often giving characters distinctive or interesting silhouettes. As mentioned above, the airplane wings on the Seekers' backs, as well as the doors on Prowl, are among their most memorable features. They look "cool" even if they are not integrated into the robot body. Thus, kibble is not an intrinsically bad thing. Most Transformers have it, and it may even be seen as an integral part of some characters' designs. Actually, if all kibble was removed from most Transformers (with the exception of shellformers), they would pretty much look like stick figures.
Other meanings / development of term
The word "kibble" was first applied to Transformers in February of 1996, when Lizard coined the term "combiner kibble" in a post to the newsgroup alt.toys.transformers. At that time, and for the next two years, the word kibble was used mainly to refer to detachable parts or accessories. It seems to have acquired its modern meaning sometime in 1998. A more detailed discussion follows:
In Google's archive of ATT, there are no recorded uses of the word "kibble" until the year 1996. In that year it was used only in reference to detachable parts and/or accessories, usually (but not always) in reference to pieces needed for combiners to combine properly, such as Scramble City feet, heads and fists. The very first instance of the word comes from a post by Lizard which is missing from the archive, but appears quoted in other posts:
Now, if only there was a convenient place to store "Combiner Kibble" -- all the hands, feet, chestplates, etc, that go into making a combiner... (Quoted text in <3122E99A.19BB@texas.net>, Feb 1996)
Said Lizard when reflecting on this ten years later, "The word comes from, of course, pet food -- I guess I felt the plethora of small pieces of Transformers detritus strongly resembled the little bits of cat food batted all over the kitchen." Lizard also used the word to refer to G1 Megatron's accessories in his BotCon 1996 report. Three other fans referred to "combiner kibble" in the first half of 1996 after Lizard's use of the term, all of them either placing it in quotes or adding an emoticon, which emphasizes the newness of the term. Later in the year the word shows up about a dozen more times, always in reference to combiner pieces or other detachable parts.
The first archived use of the word kibble to refer to something other than detachable parts is a January 1997 review of Claw Jaw, by Dave Richter. "In beast mode, he looks great from head on, but the abdomen (made out of robot legs and jointed squid kibble) doesn't look too solid if viewed from the side." (<32F023AC.397A@virginia.edu>, 29 Jan 1997) In March, Dave made a more explicitly modern use of the word in response to another fan. This post serves reasonably well as a definition:
IMO, toys should look cool from any angle. You should be able to turn a BW or any figure around have it look as cool as it does in the front. The rear should not be a place for a toy designer to throw any excess pieces on the figure that are not needed in robot mode. There are much more better designs than that.
I don't know, you've got to put the animal kibble somewhere. This is standard TF fare; look at all the T-rex bitz Grimlock has hanging off his back. BW has alot of junk hanging off the back of many of it's entries, but it doesn't overbalance them, so what does it matter? (<33237FCB.656E@virginia.edu>, 9 Mar 1997)
Still, through 1997 the "detachable parts" sense was more heavily used, with six uses to the modern meaning's three. In the previous year, 1996, the word was used to refer to detachable parts sixteen times. In 1998 Dave Van Domelen began to use the word regularly in his posted toy reviews. He used it in both senses, but more often in the modern one. Overall, the word appeared in ATT twenty-one distinct times in 1998, with thirteen of those being the new/current meaning. In 1999, usage exploded, with virtually all instances of the word being in its modern usage.
The original "detachable parts" meaning has essentially become archaic and all but unknown, most likely due to the toyline itself largely abandoning that form of construction. Indeed, it is very unusual for a modern Transformers toy not to integrate or at least store all of its parts in all modes. With regards to combiner kibble, combiner teams themselves have become rare, and the recent examples either have no kibble (Superion Maximus), or the kibble can itself combine into a separate vehicle (Universe Superion) — it's still kibble, but it's kibble with play value!
By November 1999, the meaning was well enough entrenched in ATT that Straight-Edge gave this definition, from which the meaning has not drifted at all in the years since:
It means pieces of the alternate mode (Beast mode, vehicle mode) being not only visible, but actually hanging off the robot body. IE the Doors hanging off of Prowl's shoulders, and the Crab Legs/Claws on Rampage's robot mode. They are not part of an anthropomorphic robot, hence they are kibble. (<email@example.com>, 9 Nov 1999)
It can't be determined from the limited breadth of the Usenet archive how the word kibble shifted its meaning, but it seems reasonable to guess that the earliest uses in the modern sense were simply an extension of the idea of "parts that stick on". Kibble, as we discuss it today, is just that—parts that are "stuck on".
While Transformers with no kibble whatsoever are obviously rare, one striking example would be Armada (and by inclusion, Energon) Tidal Wave. He kind of messed up, however, by being reformatted into Mirage, who has rather a lot of kibble.
Another example, but perhaps less special since he was essentially designed for perfection, is Masterpiece Optimus Prime (but see below).
There is a related phenomenon with no generally accepted name, in which a toy's robot mode has obvious elements of its alternate mode showing (usually in the chest area), which are not in fact part of its altmode. The original example of this is Powermaster Optimus Prime, who had the front windshield and grill of a semi-cab on his chest — which had no relation at all to his semi-cab mode. The false truck windows on the chest also showed up on Laser Optimus Prime and his redeco, Scourge, whose chests are on the underside of the truck in vehicle mode. Another example is the T-rex face on the chest of Transmetal Megatron from Beast Wars, which is not the same part as his actual T-rex face. Another example is the Masterpiece Optimus Prime mold, whose vehicle mode grill is tucked above the Matrix in humanoid mode. A particularly inelegant example would be the Legends-class toy for Jazz from the 2007 movie, whose robot mode "bumper" chest ends up on his roof in vehicle mode. Human Alliance Sideswipe also has the Corvette symbol on his chest which is nowhere to be seen in vehicle mode.
Its use is even seen in Transformers Animated: Bumblebee has a false roof on his chest. A more notable case is Ratchet: He has false ambulance sides on his shoulder kibble (kibble AND anti-kibble). Unlike many of the Animated toys which have somewhere to store their accessories in both modes, he also has tools that have nowhere to go when in alt mode.