Free spins bonuses give you a certain number of spins on slot games without using your own money. Most commonly, the free spins can be spent only on certain slots, selected by the operator.
Example:
If you get 20 free spins on a slot, you can play those spins for free, while any winnings are usually credited to your account as bonus money. Like other bonuses, free spins often come with rules, such as wagering requirements, which must be met before you can withdraw any winnings.