Generally, It is refund of a portion of the player's losses (over a certain period of time). Cashback amount is calculated as a percentage of the player's net losses- which is the amount of money wagered minus the amount won. Example: Casino offers 10% cashback on losses and a player wagers €/$1,000 but only wins €/$800, then the net loss is €/$200. Casino would then refund 10% of that €/$200 (which is €/$20 back to the player's account).