Short answer:
Wagering tells you how many times you must play a bonus before cashing out.
Long answer:
If a bonus has 30x wagering, it means you need to play the bonus (or bonus + deposit) amount 30 times before withdrawing. This is the casino’s way of preventing people from taking a bonus and cashing out instantly. Lower wagering is always better because you reach your cashout faster. Slots usually contribute 100%, while table games contribute less.