You remove balls from a grid, and each level also has colored backgrounds that you have to clear. The balls do not drop in by themselves: you click on an arrow at the edge of the grid to shoot a ball in.
Because the ball you shoot pushes the others in its row or column. When four or more balls of the same color end up together, the whole group is removed.
Think about where each ball will end up before you click. Building groups on purpose beats shooting at random, and it is smart to leave the edges for last because they hold the arrows. The game is a puzzle for thinkers: just you and the grid. A good companion is 2048 Classic.