The universal greeting is used as a means to express good will toward alien beings when a language barrier is present. However, it does not always work that way.
It is commonly transliterated as Bah-weep-graaaaagnah wheep nini bong.
The Transformers cartoon
The Allicons on Quintessa initially reacted well to the greeting, but when Hot Rod and Kup ran out of energon goodies, the Allicons began chanting it as they attacked the Autobots. However, the Junkions on the Planet of Junk took it rather well even without treats and threw a party, indicating that the Universal Greeting can be roughly translated into "We come in peace", "Take me to your leader", "Care to dance?", or whatever seems appropriate.
Strangely, humans had never heard of it before. Stranger yet, the rest of the galaxy is populated by robots.
Robots in Disguise cartoon
There exists a universal greeting dance, used to convince even the most implacable enemies of one's sincerity (presumably by convincing them that one is willing to humiliate oneself so thoroughly that one could not possibly harbor ill will).
Transformers Animated cartoon
The Universal Greeting has accompanying hand signals. The variants of these for five-digit species were demonstrated kindly by Wreck-Gar, who interpreted Lugnut raising his fist to deliver one of his famous punches as an invitation to perform the Universal Greeting, and reciprocated accordingly.
Stage 1: Raise the left hand, palm outward to the person being addressed. Keep the thumb tucked to the side of the palm, pointing upward. Clench the index and pinky fingers as if making a fist and extend the middle and ring fingers vertically. The gesture should be made at the same time as speaking the "graaaaagnah" part of the verbal Universal Greeting.
Stage 2: Extend the index, ring and pinky fingers. Curl the middle finger and thumb towards the palm of the hand so that they touch at right-angles, with the side of the thumb's tip pressed against the nail of the middle finger. This should be done in time with the verbalization "wheep".
Stage 3: Clench the index, middle and ring fingers. Extend the thumb and pinky fingers so that the gesture resembles a horizontally-flipped 'L'. Shake the hand sideways twice, in time to the syllables of "ni ni"
Stage 4: As the final "bong" is spoken, open your hand out so that all digits are extended and slightly splayed, as if to make a hand-print in clay. Pause momentarily and then press the thus-opened hand against that of the person you are addressing, who themselves should have a likewise open palm.
On NO ACCOUNT should the final part of the hand signals be performed if you are addressing the Decepticon Lugnut when his fist has transformed into a pressure-sensitive weapons trigger. Actually, it's probably best if you don't perform the Universal Greeting in any way before Lugnut, as any salutation short of "All hail Megatron" would be likely to incur his wrath.
IDW comics continuity
After Wheelie crash-landed on the planet LV-117, which is populated by a variety of giant monsters, he attempted to use the universal greeting on a chaosteros and lost an arm for his troubles.
War for Cybertron
While traveling to Cybertron´s core in an attempt to clean it of its Dark Energon contamination, Optimus Prime, Ironhide and Warpath stumble across a space slug, one of Cybertron's natural defenses, at which Ironhide salutes with the universal greeting.