Fill light will lighten dark objects. It is useful in the case like where you capture a photo of someone where the background is perfectly exposed but the face is a bit under-exposed. If you use the exposure or brightness setting then you will blow out the background, in this case the fill light setting will lighten the face without affecting the background (it applies to the whole image but you may only notice sections of the image being affected, like when a specific colour channel is selected for saturation)
