When you create a free account with an online casino, you usually have the option to play many of their games in free play mode. When you do this enough (if not automatically), the casino will give you a deposit free bonus back in the form of real money you can play with. This money can't always be withdrawn (it just tacks onto whatever you end up depositing at some point), and sometimes it requires that you wager the amount of the deposit X amount of times before you can withdraw it.