Wagering is the number of times a bonus has to be bet before it can be withdrawn. A €100 bonus with x35 wagering needs €3,500 of bets.
Count the turnover first
Multiply the bonus by the wagering. Compare the result with how much you usually play in the validity period. If the number is far above it, the bonus is not worth taking.
Check the maximum bet
Most casinos cap the bet while a bonus is active, usually at €5 per spin. A bigger bet can cancel the bonus and the winnings.