Rollover is the amount of times you have bet the original bonus amount. Release bonuses have rollover requirements which must be met before your bonus is released into cash funds.
For example, you may receive a release bonus with a 5 times rollover. This means the amount of the bonus must be wagered 5 times before the funds are released.
The black bar in the screenshot below will fill in with red to show the progress of the rollover requirements. You can view the status of your rollover when looking at your bonuses.
The rollover will update once your bets are settled.
Please see this example of a sports bonus with a 5x rollover requirement.