The Bonus Bank allows you to earn bonuses and keep them until you're ready to play. Here's what it looks like:
If you have an active bonus, it will show under My Active Bonus.
The total amount in My Bonus Bank can be activated to play with at any time. Simply enter the amount you want and hit 'Activate'. You will then be able to play with it immediately and convert it into real money!
If you have active bonus money, you need to play through it before activating another bonus.