You have access to 4 different types of Boost: Airstrikes call in jets to toast large groups of Creeps, Reinforcements drop in a squad of Infantry to slow the enemy down, Cryo-Bombs freeze the enemy for a short time and Fusion Cells give you extra Energy in a level to let you build more towers.
You can use your available Boosts by selecting them from the 4 icons in the bottom left corner of the screen during a level.