Remove as many squares as possible from the board. Click on a group of squares of the same color to make them all disappear in one fell swoop, causing other squares to fall into place. It is not possible to remove a single square at a time. You get far more points for clearing a larger group of squares, and also for clearing the entire board.
Swell Foop is always very quick to play, but you can make it somewhat longer by changing the board size.