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 winning in the long run, we will say it is EV+. When it is a long-term loser, we will say it is EV- and when it is neutral, we will say it is EV neutral. If you push all in with a pair of aces, you know that your equity will be (against 1 player) about 80%. Therefore, you will win the hand 80% of the time. The 80% is your equity, your winning frequency. The EV will be expressed either in dollars or in big blind.
EV+ or EV-? You have to evaluate the fairness!
A good way to know if a game is EV+ or EV- is to calculate its fairness 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 figure that you have the best hand 1 time out of EV0, the call is still very EV+. We could show you thousands of examples, but let's take a simple one. You have EV0 big blinds in games of cash games and you have JJ in your hand. A player pushes all in. Is it EV+ to call? If you call, you have to evaluate your opponent's rank. If for example your hand will have 55% equity against your opponent's range of possible hands, you know that your call is EV+.