It means the advantage the house enjoys on its game, and conversely, the disadvantage you face when playing it. A .40% advantage means the game wins .40% of the money wagered—40¢ per $100 bet, for example. Similarly, players can use this number to determine how much it will cost to play a specific game. To do this, you have to estimate how much is being bet into the stated edge, which means multiplying the average bet times the number of hands played. So if you’re playing blackjack at $25 per hand for an hour (about 40 hands with other players at the table), the equation to determine your expected result is -.004 x $25 x 40 = -$4.
Since the casino is the one with the advantage, you’re better off seeking out the games where that advantage is lowest. In your question, for example, the .40% game is better than the .56% game (where your expected loss for an hour’s play is $5.60). This is why we list these numbers when breaking down the offerings in a casino.
Also, it’s important to understand that blackjack is a game that can be played at varying levels of skill, and these percentages are specifically tied to a player who plays perfectly according to that game’s "basic strategy." For any number of decks and set of rules in a blackjack game, there’s only one correct way to play a hand based on what you have and the dealer’s up-card. That’s known as the basic strategy. If you don’t know it (or deviate from it), you’ll be playing at a loss rate that’s greater than the quoted basic strategy number.
You can use basic strategy cards at the tables, but it’s not that difficult to memorize. A good "generic" basic strategy—not perfect for all games, but close enough—can be found in Olaf Vancura’s Knock-Out Blackjack. The perfect basic strategy for almost any game you encounter can be found in Peter Griffin’s The Theory of Blackjack. And you can easily determine a game’s basic strategy advantage for yourself with Stanford Wong’s easy-to-use software program, BJEDGE.
