Exposure compensation is a way of making an image brighter or darker. The light meter in the viewfinder indicates if the light coming into the lens is too bright or too weak depending on a few factors, shutter speed, aperture and ISO. A correctly exposed image would show on the light meter with the indicator line in the middle, if its underexposed the indicator line will be to the left e.g. -1. If the image is over exposed the indicator will be to the right e.g. +1. This can be used as a technique to underexpose the background whilst keeping the foreground correctly exposed, if the image was a portrait then this technique would make the subject in the foreground pop. I enjoyed experimenting with this and I will be using it in combination with other techniques such as flash.
Light meter example