Exposure value.
A proper value would be 0 if you dial in your ISO, Shutter speed, and aperture correctly to expose the scene. You get any of these three wrong and you'll have to adjust the EV of negative or plus to get the picture back to proper exposure. This is not the way to do it however. But if you don't expose the picture correctly when you're taking it, this is a way to get the picture to look better. Other artifact will show up depending on how severe the picture was being incorrectly taken.
What camera are you using?