The house edge is the long run average percentage of each bet that the casino keeps. It comes from the gap between the true odds of a bet and the odds it pays. A lower edge costs the player less per bet, but every casino game with an edge still has a negative expected value for the player. The edge applies to the total amount bet, not to the starting money, so it adds up fast when many bets are placed. Rules such as the number of zeros on a roulette wheel change it.
Example
European roulette has a house edge of 2.70%. On 1,000 units of total bets, the expected loss is about 27 units. American roulette has 5.26%, about 52.6 units.