I kept struggling with this setting. I KNEW that it was important, but could not place it into my settings arsenal. Finally! Finally it makes sense.
If I set my camera for Aperture priority, it will automatically read the light and pick a shutter setting that will give me a 'correct' exposure. I use Aperture priority if I want to control my depth of field. If I set Shutter priority, to capture or freeze motion, my camera will set the aperture automatically for a 'correct' exposure.
But lots of times, the settings the camera comes up with are wrong, wrong, wrong. The camera is averaging the exposure according to a formula that might not be right for the light that I'm facing. I don't want to change my Shutter or Aperture setting, the camera will just pick a new aperture or shutter setting that will give me the same incorrect exposure. I don't have time to flip the camera to Manual and fiddle with setting shutter and aperture both. I want that picture and I want it NOW!
That's where the EV or Exposure Compensation setting comes in. It is the override for the average setting that the camera gives. If I snap off a shot and the camera settings gave me something too dark, I can change the Exposure Compensation to be +1 and the next shot will be brighter. For those of us that grew up with f-stops, it will be a full stop brighter. Perhaps the shot is too bright, and things are overexposed. I can set the EV for -2.0 and the picture will come out darker. Adjusting the EV tells the camera that the 'correct' exposure is darker or brighter than it would normally use.
What I must remember about using EV is to check it at the start of each photo session. The wrong EV setting could mean that all my photos are incorrectly exposed because of the light I had the last time I used my camera.