A bonus 'wagering requirement' means that you must play the amount offered and/or deposited a certain number of times, in order to complete the bonus and unlock your funds.
For example, if you received a €20 bonus with a X35 wagering requirement and a 14 day expiry, this means that you will have to play €20 x 35 (€700 in total) within 14 days to be able to complete the bonus.
As soon as the wagering requirements are met, you can then withdraw your funds.
Please keep in mind that not all games contribute the same amount towards wagering. You can check specific wagering contribution percentages from the bonus offer; or the general wagering contributions by clicking here.