The app offers an array of built-in loops and beats, however, its premise of simplistic controls and easy overdubbing suggests that users would be able to create their own background, be it by beatboxing into the mic, or by recording nearby sounds and use them as effects.
The app is free and no, there won't even be any in-app purchases to pester you. You can check out its YouTube promo vid below, however, one would be very naive to believe that what we hear in this clip has been recorded by 2 guys, hopping around like bros with a single iPhone in hand. Take it as a concept of how easy it would be to create a quick draft of a piece you've got echoing in your head, but don't expect the quality of sound to be that pristine.
Bonus points to whoever records a piece and posts it up in the comments for all of us to admire!