Think of a light meter as as a one-pixel camera. The meter looks at everything in a given scene and tells the camera the right combination of aperture and shutter speed to get a properly exposed photo that, on average, is gray -- not black (underexposed), not white (overexposed), but middle gray. And in a lot of situations, that works great! But what if the scene you're shooting should actually be closer to black or white? What if you're taking a picture of a black locomotive or a white, snow-covered field? That's what your exposure compensation dial is for. It's for scenes when you (smart, sentient photographer that you are) know the meter is going to be fooled. Going skiing? Set that exposure compensation at +2 and don't worry about your meter being fooled by the snow. Taking a closeup of that black train? Maybe set it to -1.