What is fill light?

[Thislittlelight]

Can someone please tell me what fill light is?

 

[Bitter Jeweler]

Light used to fill in shadows.

http://www.shutterbug.com/content/fill-light-use-your-flash-outdoors

 

[Derrel]

The other side of fill light is "reflector fill", which comes from ambient light being re-directed toward the subject using some type of reflecting device, as a way to lighten up shadows. Special reflectors are made to bounce light. They come in white, silver, gold, and combinations of those three. WHITE is probably the EASIEST reflector color to both use, and to find.
At times, the side of a car, shed, or house or building can be used to reflect sunlight back and toward the subject. A good example is when the sun is setting at the beach; when a person is close to a large structure, like a building, the walls of the building can act as huge reflector, and will provide what is called "fill light".

The Role of Fill