The wagering requirement (also called playthrough or rollover) is the most critical part of any bonus. Mechanically, it is a multiplier that represents how many times you must bet your funds before they convert from "bonus money" to "real cash" that can be withdrawn.
These requirements exist to prevent "bonus abuse"—where a player claims a bonus and immediately withdraws it. By requiring a certain volume of betting, operators can afford to offer these extra credits while ensuring the funds are actually used for gaming.
It is vital to check whether the requirement applies to the bonus amount only or the deposit plus bonus. For example, if you deposit funds and receive a match, a "bonus-only" requirement is much easier to clear than one that includes your original deposit in the calculation.
You can track your progress in your account settings, where a progress bar or remaining amount is typically displayed. The exact multiplier for any given offer is always listed clearly alongside the promotion in your account.