Just understand that casinos are businesses, so they add a slight advantage for themselves in all their games. It reduces the players chance to win and increases the chance for the casino will win in the long run. Basically, the casino wins more often than the players because of the house edge and these winnings are accumulated to pay out the jackpots and other promos. This mathematical advantage in the games slanted towards the casino's favour is called the house edge. Moreover, the house edge varies with the games. So high Return-To-Player(RTP) games has the lowest house edge and vice versa.