They set the odds. Say you had a game based on a 100 sided die marked 1-99, if you wanted to ensure you make money you tell people if they win you will double their money, but they have to get a 51 or higher. In this case 51 or so percent of the time you the house wins, and 49 percent of the time the player wins.
Over a large enough amount of time you will win. You can better your odds by limiting the bet amount, this will make it so people that win big have to keep winning bug to double their money which is progressively unlikely since the odds are in your favor.
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