This app is a kind of musical toy to explore and play around with harmonies and rhythms. The three layers or tracks are synchronised to a global harmony and tempo, so its easy to create "good" sounding patterns without musical knowledge. This is the default setting, but its possible to adjust most of this stuff to get some more interesting and directed results.
In general it works a bit like an arpeggiator. You define a scale and octave range and rhythm (or just use the defaults), create some visual shapes on the grid and the notes are assigned semi randomly.
Tapping on the grids activates the build mode for one layer.