Ambient light
Soft, even light from everywhere, so no side of a shape falls fully into black.
See it in WebGL playground. Under Lighting, turn Key, Fill and Rim down to zero and leave Ambient on.
In plain words
Light bounces off walls, sky and ground, so it reaches even the sides facing away from a lamp. Ambient light is a cheap way to copy that bounce. It adds the same small amount of light to every surface, from every direction. With only ambient light, shapes look flat, because nothing is brighter or darker. Mixed with a main light, it keeps the shadows from turning pitch black.