Click on play. A short tutorial will resume. Your objective is to clear and "plop" all the water drops from the board. You need to tap the water drops to make them explode and send smaller drops to the other drops, causing them to swell and eventually "plop". You can fill drops by yourself, but it costs you a drop every time. The amount of drops is show on the right hand side.
Click on "tutorial" (right side menu) if you need to see an example again of how to play.