Well for an atmega328 at least I'd imagine it's the same thing that happens if you're not holding the reset button down on an arduino board, or what's described on page 3 of the atmega datasheet. If the controller is powered on and you're not resetting it, then what's the controller doing? I'll leave that as an exercise to the reader... I know very little about electronics and that was easy enough to work out - you should probably learn how to at least do this kind of research if your goal is to be an effective employee...