In a normal casino bonus, you can only make a withdrawal after completing the wagering requirement. In a forfeitable bonus, you are allowed to make a withdrawal before completing the wagering, but only under the following circumstances:
You started playing first with your real money (from the deposit)
You have some wins using real money bets.
In this instance, you don’t have to wait until you use up your bonus money and then complete the wagering requirements. You can straightaway withdraw your “real” money winnings. But this option is NOT available if you used your bonus money first.
Forfeitable bonuses were rare in the early years of online gambling. But they have become a lot common in recent years at UK online casinos in particular, after the UK Gambling Commission rule changes.