Fill Light

A fill light is used to reduce the contrast ratio of a scene by adding light to the shadow areas, effectively making the dark areas brighter or "filling" the dark areas with light. There are times when you don't want to see any detail in certain shadow areas, but if your lens is set to an exposure which will accurately render the highlights and you still want to see some detail in the shadows, then you will need to add a fill light.

This can be done by using a soft light or a bounce light because you generally don't want to add extra shadows from a hard light source.