it's about probability.

imagine I have a dice, and I offer you a $1 bet. if you roll your number, I pay you $5.

lets assume this is a fair dice, so on average each number comes up as frequently as each other.

what this means is that theoretically, if you place six bets, you will win once and take the $5 prize. but you will have bet $6. so I will be up $1.

this is how gambling works as a business but with actual dice instead of theoretical ones.

it is always possible that on any one bet you will win and take my money.

but I know that the more bets I take, the closer the outcome will be to the theoretical average, and I will make my margin just as in the dice bet.

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