Enso looper is a big, big deal for iOS musicians. In principle, it works like any other looper app or hardware looper: You play music into it, and then that music is looped over and over, forming a backing track for more playing.
But Enso is hot, hot, hot for two reasons. One is the amount of control you have over the looping. The other is that it is an Audio Unit, which means that it can be used inside other apps, like GarageBand and AudioBus.