Ambient light, also known as diffuse environmental light, is light that is present all around the Scene and doesn’t come from any specific source object. It can be an important contributor to the overall look and brightness of a scene.
Ambient light can be useful in a number of cases, depending upon your chosen art style. An example would be bright, cartoon-style rendering where dark shadows may be undesirable or where lighting is perhaps hand-painted into textures. Ambient light can also be useful if you need to increase the overall brightness of a scene without adjusting individual lights.
Ambient light settings can be found in the Lighting window.