You are telling the camera to over expose the picture by two f stops from the exposure the camera would normally use. You do this when you are trying to photograph something that is above average brightness when the camera would under-expose the scene, e.g. a dark subject against a bright white background, the camera would take into account the white background and stop the lens down, result an under-exposed image, exposure compensation , as this is called, prevents the camera from using the "wrong" setting and you end up with a correctly exposed image, how much compensation you need to apply depends on the individual scene, i.e how much white is affecting the result, the converse is true for dark scenes where the camera will try to over expose the scene, in this instance you need to choose a minus exposure compensation value. Using flash is just part of the exposure, it is up to the camera to decide how much flash to emit depending on the chosen exposure saetting.