What Is the House Edge
Casinos are businesses, and they wouldn't be able to keep running at all if they didn't make money. If they just paid out money to everyone who played so that these players just won every time, then they wouldn't stay in business at all. Even paying out an amount much lower than frequently would tend to lead to their financial ruin. Instead, casinos rely on what's called the House Edge, which means the advantage the casino has over players.
More specifically, this tends to be defined as the profit the casino makes as a percentage of the player's bet in the casino. Another way of putting this is to say that the edge refers to the ration between the average loss to the original bet, or in other words, the statistical advantage a casino has over a long enough time period.
This ensures that in the long run, a casino will make a profit even if it loses some money now and again while players win. After all, the promise of winning is the only thing that will draw players to the casino in the first place.
Staying Ahead of the Edge
The important thing about the Edge is that it mostly only applies if you keep playing the same game at the same casino. It's just like any game of chance, you can flip three heads in a row, but if you keep doing it, eventually it will be evened out with about the same number of tails. This could be over a long period though, is this is kind of the space that individual players need to try to live in.
However, it does mean that you have a chance of winning in the short term, this is what everyone relies on when trying to win these games of chance. Someone is going to lose in order to feed the house, but it doesn't necessarily have to be you, is the idea.
The Edge and Online Casinos
This matters in particular with online casinos because it's easier to shop around for the casino you wat this way. In other words, you can find a casino that has less of a house advantage. If you're choosing to gamble, then it makes sense to find the lowest disadvantage that you can.