The term is simple: House Edge is the difference between the probability of casino's win and player's win. If I say, house edge is 2% then the probability that result of the bet will in favour of house is 51% and that of player is 49%.

Let's take a simple example of Dice game. Suppose a dice game has house edge of 2% then it will pay you 2.00x for rolling above 51 or below 49. Ideally, it should be 2x for rolling above 50 or below 50. But due to house edge factor, the possibility of you winning the bet as a player falls from 50% to 49%.