EV stands for expected value. It refers to whether your decisions to call, raise, fold, whatever, are profitable in the long term.
For example, if you have A-A, and you raise, and someone ships, when you call, you have an EV of roughly 80%, if heads up. So 80% of the time, we expect to win the pot. 20% we dont. This is just an estimate based on incomplete information.
Another spot would be if you have a flush draw on the flop. We would have roughly 36% +/- equity on the flop against any 1 pair hand, if we get it in and see turn and river. So we are roughly a 2:1 dog here, so our pot odds must be better than 2:1 to have +EV.
So example, we are out of position, we raise to $20 with A-2hh. 1 caller. Flop Kh-7h-10d. We bet $25 (pot is $40), we get raised all in. Villain has us covered. All we really have is the flush draw. We had $200 starting hand. Now we have $155 in front of us.
So villains bet is $180, there is $25 (our flop bet) plus $40 (pot from preflop). This equals a grand total of $245 dead money in pot. We have to call $155 to win $245. So we aren't even getting 2:1 pot odds. If we called here, this would be negative EV. This means if we ran this hand a million times, same scenario, we expect to lose more than we win.
Catching my drift?