A wagering requirement (also known as playthrough or rollover) is a multiplier that determines how much you must bet before bonus funds can be withdrawn as cash. For example, if a bonus has a requirement, you must bet the bonus amount multiplied by that figure before the funds are unlocked.
These requirements exist because bonuses are provided as a service to enhance gameplay; without them, players could simply claim a bonus and immediately withdraw the funds. There are two main types of wagering: Bonus-Only (where only the bonus amount is wagered) and Deposit + Bonus (where both the deposited amount and the bonus are included in the calculation). The latter effectively increases the total amount you need to bet.
You can track your progress in your account dashboard, which shows the remaining amount required to meet the goal. The exact multiplier for any given offer is always displayed clearly alongside the promotion in your account section.