Ev is expected value.
consider a hand where you have a naked flush draw and your opponent has tptk on the flop. You have a roughly 36% (~1.9:1) chance to win, so when faced with a pot-sized AI bet (giving you 2:1 odds), you can make a +EV call. If your opponent moved in for 2x pot(giving you 1.5:1 odds), it would be a -EV call.

Calculators range from single hands to entire databases full of hands. If you get specific with them (using pokerev for instance) you can fix some leaks in your game. Look up spoon's post on filtering in pokerev. That will give you an in-depth idea of how this stuff works.