The Time Window was a triangular device of Quintesson design that opened a window into time...what did you think it was?
Extended use of the machine is very dangerous, as it unleashes chronal energy into the universe, which affects time itself. It can cause the past and the present to collide with each other, creating numerous temporal anomalies, such as regressing an adult into an infant or making two duplicates of the same individual. If the process is not reversed, then the entire universe will cease to exist. Temporal mechanics 101.
The Transformers cartoon
The Quintessons built a Time Window on a remote asteroid with the attention of mucking up the timeline by capturing A3 before the Autobot rebellion entered full swing. The intention, of course, was to prevent the Quintessons from abandoning Cybertron to the Transformers, thus ensuring that they remained in control.
Unfortunately, as was the case with most evil plans, the Quintessons' experiments attracted Autobot attention and their base was attacked. This resulted in Wreck-Gar, Blaster, Ramhorn, Rewind and Blurr getting lost in the past...more specifically, on Quintesson-era Cybertron, where they aided their ancestors in rebelling against their tyrannical rulers.
After the device fell into Autobot hands, it was left on, sending chronal ripples into the universe and causing space and time to collide. The Quintessons, after the hysterical outcry of their chief scientist, attempted to regain control but were beaten back. The scientist then attempted diplomacy and urged Rodimus Prime to deactivate the machine, though he also attempted to trick the Autobots into ensuring that A3 did not return back. However, A3 ran into the Window, retrieved the Autobots, and went on to play his part in history.
Once the Autobots were returned to their present, the Time Window just blew up. If only they'd had Scotty there...