A wager is a coefficient that determines how many times a player has to play through the casino funds after using up a bonus.
Imagine a casino has given you 20 bonus free spins with a wagering requirement of 35x. Let’s say that after using up all the free spins, your winnings are $30 casino credits. You can’t withdraw them yet: your next step is to play them through 35x times. It means you need to wager the amount of $30x35=$1050. After doing it, you will be able to withdraw your winnings.