Definition of EV
When we talk about EV, we are talking about Expected Value (long-term gains and losses). The expected value can be negative or positive depending on our equity. When a game is a long term winner, we say it is EV+. When it is a long term loser, we say it is EV- and when it is neutral, we say it is EV neutral. If you push all in with a pair of aces, you know that your equity will be (against only 1 player) about 80%. So you will win the hand 80% of the time. The 80% is your equity, your win rate. The EV will be expressed either in dollars or in the big blind.
EV+ or EV-? Equity must be evaluated!
A good way to tell if a play is EV+ or EV- is to calculate its equity and relate it to the size of the pot. To take an extreme example, if a player on the river bets 1 chip in 1 million chips, even if you estimate that you have the best hand 1 in 100, the call is still very EV+. We could give you thousands of examples, but let's take a simple one. You have 100 big blinds in cash games and you have JJ in your hand. A player pushes all in. Is it EV+ to call? If you call, you need to evaluate your opponent's range. If, for example, your hand will have 55% equity against your opponent's range of possible hands, you know your call is EV+.