Some free spins offers may include a wagering requirement (for UK players this is less of an issue since the introduction of Bonus spins where winnings are not subject to wagering!)
The amount of wagering required is generally listed a something like 40x, 30x, 20x your winnings, this means once your free spins have been completed the winnings from them must then be played through on other slots or valid games a specific amount of times depending on how much you won also!
A simple example would be that you won €10 from a free spins offer, the casino states that this needs to be wagered 40x times, this means that in total you need to wager through €400 (40 x 10) in value in regular spins before these winnings are unlocked, converted to real cash and fully withdrawable.
Thankfully most online casinos are a lot more transparent on wagering conditions now and allow you to track wagering rates visually generally with a percentage bar showing how close to completion you are.