No deposit free spins can also be called registration spins. When you sign up in an online casino they will award you with a few rounds on a selected slot machine. The winnings from these free rounds are then turned into bonus money. With the bonus money you can play any game you want, however you can not yet withdraw the winnings. For the money to be truly yours you will need to clear the bonus. There are so called wagering requirements; the amount of times you need to wager* the bonus money for it to be yours. There might be additional terms and conditions so always make sure
You get 20 free spins on registration. With the free spins you win an amount of €5. The wager in the casino you play is 40x. This means you will have to wager in total 40x5 = 200€. With the €5 bonus money you keep playing and you manage to hold on to it and even win some more. After wagering the €200 you end up having €25 in your casino account. This is now your money and you are allowed to make a withdrawl.
Please note that the wagering often has a max bet (mostly €5 / bet) and certain games are excluded from wagering or do not fully contribute. Again: make sure to read the T&C inside the casino to know the bonus rules.
*wager: A wager is a bet you make. After you make a bet of €1, you have wagered €1. After making 10 bets of €1, you have wagered €10.