The reamp unit is a box that records the signal from your guitar (or bass or vocals) instead of recording the signal from a mic.
So after you record it you can then play that signal thru different amps with different settings, in different rooms etc . . . (one just turns the unit around)
So the other night my bro dubbed over a song with 2 signals. The first one was from a mic and the second was the reamp .wav
We played back the reamp .wav file (it was weird to listen to a guitar thru a cab with no guitar player haha) and solo'd out his guitar. We were able to position the mic in the best area, unsolo it to the rest of the mix, and then adjust his EQ and gain till his guitar sounded best. Way cool.