Using the word “free” in this context is probably a bit misleading because a free cash bonus isn’t necessarily free! Let’s say that you have claimed a £10 no deposit bonus with a wagering requirement of 50x, This indicates that you need to wager £10 x 50 = £500 to be eligible to withdraw your £10 bonus from your account.
That’s a lot of money to wager for an average player, but it’s important to remember that you’re still essentially getting £10 to make bets with (win or lose!), which is better than nothing. You will have to bear in mind that the higher the bonus the higher the wagering requirements typically are.