A rollover is the amount of times you need to 'play through' a bonus in order for you to be able to withdraw it.
The rollover amount is expressed as a multiple of the bonus awarded.
For example, if you deposit $200 and receive a $200 matched bonus with a 35 times rollover, this means you must stake $200 x 35 = $7,000 in order to be able to 'release' your bonus winnings and withdraw them.
Once you are credited with a bonus you will be able to see it as a separate 'Bonus balance' next to your Cash balance.
You can also see how much you have contributed towards the rollover in the My account section of the website.
Once you have reached the rollover requirement, your bonus balance will be transferred to your Cash balance and you can withdraw it. Note that you are unable to withdraw funds from your cash balance while in a bonus
Finally, please refer to the individual "Terms and Conditions" of any bonus offer, as these may have stipulations on the amount that certain games can contribute towards your rollover, or limits on the amount of the bonus balance that can be transferred into your cash balance once the rollover requirement is met.