You can play on it, it's a part of your lost money, basically it's a chance to win back your loss. Read the rules in the terms of the casino, I play here Avantgarde Casino. look at the percentage of cashback, there are good conditions. You can withdraw if you win it back. Basically, the casino uses this so they don't lose the player if they are unlucky this time. This is the best casino in Australia I know