The Wagering Requirement (also known as playthrough or rollover) is the most important term in any bonus offer. Mechanically, it is a multiplier that determines how many times you must bet the bonus funds before they turn into withdrawable cash.
Why does this exist? Bonuses are marketing tools. If players could withdraw a bonus immediately, it would be a direct cash payout. The requirement ensures the bonus is used for its intended purpose: playing the games.
There are two main types of requirements: Bonus-Only, where only the credited bonus must be wagered, and Deposit + Bonus, where both the deposited amount and the bonus credits are included in the total wagering target. The latter results in a higher total amount to bet. You can find the exact multiplier and the total remaining wagering amount in your account's promotion tracking bar.