When you make a deposit, you can receive a bonus as a gift. The deposit is credited to the real money account, and the bonus (gift money) is credited to a separate bonus account.
You can't withdraw the bonus, but you can play with it. Money can only be withdrawn from the real account.
In order for the bonus money to be withdrawn, you must wager an amount equal to:
Bonus Amount × Wager
The wager is a special multiplier that determines how much money you must wager before the bonus money becomes real. It is indicated in the bonus description.
After that all the bonus money is transferred to a real account where it can be withdrawn. You can see the progress of your bonus wagering in your profile.
If you withdraw your winnings before that point, the bonus account is reset.
For example
You made a deposit of 50 AUD and received a 50% bonus with a wager of 30.
Your real account is 50 AUD and the bonus account is 25 AUD. You can play with 75 AUD. The 50 AUD amount is available for withdrawal.
In order to cash out your 25 AUD bonus, you must wager (not deposit!) 25 AUD × 30 = 750 AUD.
After that, the full bonus amount will be available for full withdrawal.