In poker, expected value (EV) tells you how much money you expect to gain or lose on average from making a specific play. It is calculated by working out all the possible scenarios that may occur after your action, and then adding up the amounts that you expect to win or expect to lose with each possible outcome, and then weighting the result by the percentage probability of each outcome.
The classic explanatory example is flipping a coin. Let’s say that you have a friend who foolishly offers you $2 every time the coin comes up heads, and you only have to pay him $1 when it comes up tales. Your EV in this situation would be + 50c. Half the time you would lose $1, and the other half you would win $2, resulting in an average profit of 50c for every time you flip the coin. Correct poker strategy requires an understanding of EV.
This poker video tutorial discusses the importance of knowing the long term probabilities of a certain event to help you find +EV situations in poker.