A cashback bonus is a form of perk, incentive, or reward that an online casino will offer to its players who have lost money in their wagers. It's essentially a 'refund' in that the player will receive a percentage of his or her losses back from the casino. The percentage amount can vary and is typically based on the amount of money spent and lost. It's important to note that not all online casinos offer a cashback bonus, so be sure to check the individual casino for eligibility requirements.