EV, or expected value, is what the player could expect to win or lose per a single game if he was to place the same bet in the same situation an infinite number of times. Formally, the expected value is a weighted average of all the possible values. In simpler terms, every possible random outcome is multiplied by its odds and the results are combined to find the Expected Value.
Betting on Red in European Roulette gives you the odds of 18:19, as the wheel has 18 Red squares, 18 Black squares and a single Green square for a total of 37 squares. As a result, the expected value of a $1 bet is −$0.027, since (18/37*1)+(19/37*-1)= (-0.027).
In a casino setting, the concept of expected value is usually applied to entire casino games. Poker players on the other hand tend to apply this concept to calculate the correct course of action in every possible situation. Consequently, when a poker player says that a move is EV+, it means that the expected value is positive and therefore it’s advantageous to perform it under almost all circumstances. EV- on the other hand means that performing the action is going to cost you money in the long term, so you should generally avoid it.