Each time you match candies, the bottle at the bottom right of the screen will fill slightly. When the bottle has filled completely, the Boost is crafted and you can tap it to use it. Each Witch crafts a different Boost:

Candice's Color Pop will remove the most common color on the board and can even make Bombs.

Trixie's Lucky Shot will target up to 7 candies or objects that will further the levels objective.