'Boosters' are enhancers that will help you answer questions you don't know correctly. Once you activate them, the right answer will automatically be selected. You can find them on the lower part of the screen during the game.
When you start playing, you will get three boosters for free. You will be able to see the number of boosters you have on the upper corner of your main screen.
Once you use them, you will be able to buy more at the game's Shop.