A wagering requirement (also known as playthrough or rollover) is a multiplier that determines how many times you must bet your bonus funds before they convert into withdrawable cash. These requirements exist to prevent players from claiming a bonus and immediately withdrawing it, which would make the bonus unsustainable for the operator.
There are two main types of wagering: Bonus-Only, where only the bonus amount is subject to the multiplier, and Deposit + Bonus, where both your initial deposit and the bonus are locked until the requirement is met. The latter is significantly more restrictive as it ties up your own cash.
You can typically track your progress via a progress bar in your account. To find the exact multiplier for any offer, always look for the "Wagering" or "Rollover" section in the specific offer's terms and conditions before clicking "Claim."