The house edge is the built-in advantage that casinos have on most of the games played on the gaming floor. Casinos are in the business of making money: they do this by ensuring most of the games played on the floor are weighted in the casino’s favour.
For example, in American roulette, the house edge is 5.26%. That means that on average for every $/€1 you bet on the roulette wheel you will get 95p back as winnings. For European roulette the house edge is different, as because there is only one zero on a European roulette wheel, the house edge is lower at 2.7%, meaning you will get back 0.973 on your $/€1 bet.
Not all games have a house edge. In poker, for example, the casino acts as dealer and does not participate in the game. Instead they take a small percentage of the pot called a “rake” (usually 5-20%), or they charge “time” which is a fixed fee for every half-hour of play.
The house edge works over time. If you bet $/€1 at the casino you are not going to get back 0.95p as winnings every time. You are either going to win or lose the whole amount.
Statistically you have to play 100,000 times before your win averages are near to the house edge. If you were to play roulette 30 times a day, every day, it would take nearly 10 years to complete 100,000 games. That doesn’t mean that you would be down on money for all of that time – you could just as easily be up. It all depends on your luck and skill at the casino’s tables.
In some games, the house edge swings to the player for a certain period of time. Blackjack is a prime example. When the deck is full of high cards (aces and 10s), statistically the player has advantage over the dealer. The trick is knowing when the deck is full of high cards. This is what “advantage players” such as “card counters” are trying to achieve. However, very few of them pull it off.