When you receive any bonus, this will lock some (or all) of your withdrawable balance until the bonus wagering is completed and add your balance to the bonus cycle. You can access this information when you click on the screen showing your balance next to the My Account section.
If you only have a bonus balance in your account, you can withdraw your bonus balance and winnings from bonuses by completing the wagering of your bonus.
If you have an active bonus in your account, you can go to your "My Bonuses" page and access the details about the wagering requirements.