The leverage dictates how much of your bankroll that you're willing to risk per a bet.

If you had 10 STEEM invested at 99x leverage (max allowed) and a player came in and won the most they possibly could in one roll you'd lose 99% of your bankroll.

Money comes from the loses of gamblers. Essentially the leverage allows you to risk more of your investment in order to get more of the gamblers loses, but also makes you more likely to get your capital won by gamblers betting and winning if they get lucky.