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%. In other word, for every 100 bets made on casino, the result should be in favor of house 51 times and in player's favor 49 times. However, in short run the results can be significantly different from the expected house edge. Let say, a single player made 100 bets then it is possible that player wins 54-60 bets while only 40-46 bets goes in the favor of casino. But in long run, the result will automatically diverge in the favor of casino. Let say, over 2 million bets are made on the casino then approximately 51% will go in the favor of casino.