Making a song is easy. Each disc has its own sound. By placing the discs on the squares, you can hear them. Add some discs, take some away, move them and hear how the sound changes. What you’re doing here is sampling. You take existing sound clips and compose your own hit with them.
Nowadays, there are lots of ways to compose your tunes. In this case you are laying discs, but it can also be done, for example, by dragging instruments around on the computer. The possibilities are endless.