Exposure Compensation is a feature on a digital camera that allows you to adjust the shutter speed measured by its light meter. Usually, the range of adjustment goes from +2 to -2 EV in 1/3 steps.
Lets say that you point the camera at you subject and the meter says you need 1/250 sec. shutter speed at aperture 5.
If for whatever reason you select +1 exposure compensation, the shutter speed used by the camera will be 1/125 sec.
If you use -1 exposure compensation, the shutter speed will be 1/500 sec.
Every increment in compensation (+2 | +1 | -1 | -2) increases or decreases the amount of light going through the camera by a factor of 2.