Boosters are significantly more powerful bubbles that can help in tricky situations. They can be obtained with coins or by completing quests and achievements in the game. Your booster inventory is shown below the slingshot.

You can use a booster by simply tapping the picture of the one you would like to use, and it will be added to the slingshot. If you change your mind, tap the booster icon again, and it will return to your inventory. However, if you run out of some booster, you can purchase more by tapping the booster icon.