A forfeitable casino bonus has terms that allow you to use your qualifying deposit amount first. If you win any prize, you can withdraw it immediately and forfeit the bonus amount. You can start playing with the bonus only if you lose your entire deposit.
You also have the option of starting to play with the bonus amount. If you do so, you have to satisfy the bonus wagering requirements before cashing out your winnings.
Forfeitable casino bonuses were available only at reputed and licensed online casinos. However, most online casinos offer them now because the UK Competitions and Markets Authority (CMA) forced them to change their bonus policies.