There are two types of boosters that can help you on the levels, pre-session boosters and in-game boosters.
Pre-session boosters can be obtained by watching a short ad before retrying a level, or by selecting them on tournament levels. By watching ads you can get one free booster to start the level with. The boosters available are Coral, Block Smash and the Rainbow Fruit. Block Smash causes damage to random blocks on the level, while the Rainbow Fruit will destroy all of the fruit of same color that it is swapped with. On tournament levels you can select three boosters to start with. Boosters available on tournament levels are Triple Coral, Double Bitelings, Octo, Rainbow Fruit and Block Smash.
In-game boosters include Smash Crab and Octo Strike. These boosters can be purchased with coins. To select the booster, tap the booster button at top right in the level. To de-select the booster, tap the button again. Use Smash Crab to destroy any target or pile of targets on one tile. Octo Strike crushes the target and adds an Octo to the tile you targeted.