In plain english: It is a small computer that reads the trigger and controls when and how the motor turns on and off - it can do things like burst fire, motor speed reduction, low battery warning, and so forth. Since an AEG's guts is really just turning a motor on and off - controlling that part with a computer allows you to improve and do a surprisingly large and interesting list of things! There is a catch though, you need to re-wire your gun to install it (so it requires gun tech skills.)
You can visit the site tunabreath mentioned or my
ASC retailer post to know more.