As the name suggests, a no-deposit free bet is a bet that doesn't require you to pay for the stake. However, if your bet comes through, you can still keep your winnings (subject to the Casino's terms and conditions).
99% of all free bet promotions will have wagering requirements attached. A wagering requirement is the amount of money you must wager with your chosen sportsbook before the bonus funds can be withdrawn as real money into your bank account or e-wallet.