A universal emulator was a part of a Transformers anatomy which some members of the race were equipped with. In the only known example, it was located behind the insignia of the Transformer in question, Perceptor.
The device's name suggests that it can emulate the functions of other pieces of technology, and thereby replace them in other mechanisms when the required piece is unavailable. The extent of the emulator's abilities is unknown; in the only known instance of this ability being put to use, it emulated the function of an isolator key. Additionally, when the emulator was extracted from Perceptor's body, his personality went with it, and inhabited the mechanism into which the emulator was inserted. Whether or not this is a universal feature of the emulators, or if it was unique to Perceptor's emulator alone, or if it was an unforeseen occurrence that may or may not have been due to the unpleasant circumstances of its removal, is unknown.
In case you didn't realize, we really don't know much about this thing.
The Transformers cartoon
Upon entry into Quadrant X, Perceptor was damaged to the point that he was trapped in his alternate mode. Crashing to planet Zamojin, Katsu Don, one of the native Zamojins, carved off Perceptor's Autobot symbol, unwittingly taking the universal emulator out with it, to provide a face for the long-inert robot known as Nijika. The emulator promptly interfaced with Nijika's systems, transforming the Autobot symbol into a functional face, and allowing Perceptor to operate the robot.
The Quintessons were also after this piece of Perceptor, seeking to replace the damaged isolator key that operated the Quadrant Lock, a device which kept the planet's populace deprived of its power-giving starlight.