The wagering requirement (also called "playthrough" or "rollover") is the most critical part of any bonus. Mechanically, it is a multiplier that determines how many times you must bet the bonus amount before those funds are converted into real, withdrawable cash.
These requirements exist because bonuses allow players to play with the house's money. The rollover ensures that players engage with the games and do not simply deposit, claim a bonus, and immediately withdraw the total amount. Without this, the sustainability of bonus offers would be impossible.
It is important to distinguish between Bonus-Only and Deposit + Bonus wagering. In the former, only the bonus amount is multiplied. In the latter, the multiplier applies to both your deposit and the bonus, meaning you must wager a significantly higher total volume to unlock the funds.
You can track your progress in your account's bonus menu. The specific multiplier for any active offer is always clearly displayed alongside the promotion in your dashboard.