UFO is a booster in Candy Crush Saga that is used in all levels. It is unlocked at level 21. Each usage costs 20 gold bars. Using this booster will summon a UFO that puts 3 wrapped candies on the board (typically near blockers, jellies, or under ingredients) and sets them off.
Unlike the special candy version, this UFO will arrive from outside the screen, and will take action immediately.