I don't know how basic you're looking for but here goes. EV of course stands for Expected Value
. The simplest way to explane EV would be to say that any decisions you make that are mathematically profitable are +EV and any that mathematically unprofitable are -EV.
So lets say you have a coin and someone gives you 1:1 odds
to pick whether the coin will flip heads up or tails up. Because the chance of either outcome is 50%, this would be a neutral EV proposition if the coin flip were played out an infinite number of times. That is, you would neither make nor lose money. A pointless and boring game indeed.
But now lets say someone if foolish enough to give you 2:1 odds on your choice of heads or tails. Well, this hasn't changed your chance of winning a bit, you still have 50% equity before the coinflip. But it has changed the game from being neutral EV to +EV for you and -EV for the chump who made the bet. On an average you'll still only win the coinflip 1 out of every 2 times but now your wins will be twice as big as your losses IF and only IF you play the scenario out an infinite number of times.
I say infinite because any finite number of flips will result in someone being either slightly ahead or slightly behind their true expected value.
This is a poker forum
so I suppose you want to know how it applies to the game. Every decision you make in poker is either profitable +EV or unprofitable -EV. Sometimes the math is straight forward like when someone bets 1/3 pot on a turn card where any heart will give you the nut flush. In this case there are 46 unseen cards and 9 of them will make the winning hand for you. You're being laid 4:1 pot odds
to call but you only need 3:1 to make it profitable, so you call. You will still lose the hand 3 out of 4 times but the eventual win will more than make up for the losses. This is exactly like the coin flip scenario from above except that while no sane person would ever give you 2:1 odds on a coin flip they will give you 4:1 odds or better in poker on a draw. It happens all the time, either because they don't put you on the correct hand or because they just plain don't know how to play.
Now as far as poker goes the example above is about as straight forward as it gets but the actual opportunities for finding other less obvious +EV situations are almost limitless. Sometimes they can be pretty abstract like estimating someone bluffing range on the river to determine whether to fold a Full House or call with Ace High. And sometimes it's real easy like when your counting outs to make the best hand. Or, sometimes you might even intentionally make some small -EV decisions in order to get a bigger payoff on the +EV ones. Now you're playin' poker!
OK, enough for now. Hope that wasn't too confusing.