By clicking the bubbles you increase them by the added drops that you have on stock (the number in the upper right corner). As soon as the bubble reaches critical value it breaks and enlarges bubbles around it. The point of the game is to crack all the bubbles while the number of available drops remains for the next levels. However, this many not seem that way at first, the lack of drops will add to the difficulty of the game and you will have hard time just getting to the next level.