8BITar Hero takes audio generated from a NES game run on an emulator and runs it into a "Rock-Band game" with up to four separate music tracks.
If that isn't straightforward enough for you and the video isn't doing it either (or, like me, you're in class where you shouldn't be watching YouTube vids), here's the description from the actual site:
8BITar Hero is basically Rock Band except with a Nintendo emulator generating the music and levels.
Gameplay levels that are procedurally generated from the act of playing a game. One person plays NES games on an emulator, others play a Rock-Band game with levels that are generated algorithmically from the audio code of the NES game. The focus here is the generative nature of the game, and the process by which the levels are created.
No, you can't use your plastic instruments to play 8BITar Hero — but run Mega Man 2 through the emulator and you'll really be giving your hand-eye-coordination a workout.