The wagering requirement (also known as playthrough or rollover) is the most critical component of any bonus. Mechanically, it is a multiplier that represents how much you must bet in total before bonus funds—and any winnings derived from them—become withdrawable cash. For example, if you have a bonus and a specific multiplier, you must place bets totaling that multiplier times the bonus amount.
Why does this exist? Without wagering requirements, players could claim a bonus and immediately withdraw it, which is unsustainable for operators. This mechanism ensures the bonus is used for its intended purpose: playing the games. Some offers apply the multiplier only to the bonus amount, while others apply it to the deposit plus bonus. The latter significantly increases the total amount you must bet before cashing out.
You can track your progress in your account dashboard. The specific multiplier for every active offer is clearly listed alongside the promotion in your profile or within the specific offer's terms and conditions link.