In every level there are several polygonal fragments. They can make the square and fill up the empty square vessel at the center. When it's done the level is complete. The number of fragments gets higher from 5 to 14.
Fill the square vessel at the center with all the fragments that fall down at the start of the level.
A context menu should be shown when right-clicking or, on touch devices, long-pressing in the game. This menu especially allows to play in full screen and control the volume.