If you attempt to withdraw money before fulfilling the wagering requirements, the casino won't allow it. Wagering requirements, also known as "playthrough" requirements, are conditions set by online casinos on their bonuses and promotions to avoid potential misuse.
Bonuses usually come with wagering requirements, which means that you have to bet a certain amount of money before you can withdraw any winnings that you make with the bonus money. Wagering requirements can vary widely from one casino to another, so it's always important to do some research.