The wagering requirement (also known as "playthrough" or "rollover") is the most critical part of any bonus. It is a multiplier that determines how many times you must bet your funds before the bonus balance transforms into withdrawable cash.
Why does this exist? Because bonuses are promotional tools, wagering requirements prevent players from simply claiming a bonus and immediately withdrawing it. It ensures the funds are used for their intended purpose: playing the games.
There are two main types of calculations: Bonus-Only (where only the bonus amount is multiplied) and Deposit + Bonus (where both your deposit and the bonus are multiplied). The latter results in a much higher total amount you need to bet before withdrawing.
You can track your progress in your account dashboard. The exact multiplier for any specific offer is always displayed clearly alongside the promotion before you accept it.