The objective of the game is to clear the blobs (those water droplets) off the board. To do this, you'd need to selectively fill certain blobs until they burst, causing their droplets to go floating towards neighboring blobs causing them to burst as well.
You start off the game with only 10 drops and are rewarded with an extra drop when you beat a level. Every three blobs involved in a combo also gives you an extra blob. Make sure to use your drops wisely, because using them all up means 'Game Over'.