Power ups in candy crush are used as an extra help to pass levels.They basically can be used to simplify the gameplay. This is done in various forms. Some boosters are activated via the screen before the level starts, others can be activated during the gameplay, while some are used as a consolation to prevent failure and encourage retrying.
There are various boosters in the game and all of them have different properties. Boosters come with a certain number of charges. Once you use the booster, one use is consumed.
Jelly fish, coconut wheel, and lucky candy boosters may only be used in jelly, ingredient, and candy order levels respectively. Jelly fish and coconut wheel boosters may also be used in mixed levels. In-game boosters may only be used if the conditions can create any effect: for example, you cannot use the Bomb Cooler when there are no bombs, and you cannot use the Bubblegum Troll when there are no chocolate fountains.