An important contributor to the overall look and brightness of a scene is ‘ambient lighting’. This can be thought of as a global light source affecting objects in the scene from every direction.
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.
Without using one of Unity’s precomputed lighting solutions, ambient light will not be occluded and therefore will not be physically accurate. However if either Baked GI or Precomputed Realtime GI are enabled in your scene then this ‘skylight’ will be blocked by objects in your scene - giving a more realistic result.
The same scene with no light whatsoever (Left) and with only Ambient Light (Right). Notice how the visible Skybox does not change when changes are made to the Ambient Intensity.
Now using Precomputed Realtime GI by marking the objects as static. Notice how light is now occluded in areas of contact between surfaces.
A significant advantage of using ambient light is that it is cheap to render and so particularly useful for mobile applications where perhaps it is desirable to minimize the number of lights in your scene.
Ambient Lighting can be controlled in the Lighting window from the ‘Environment Lighting’ section (Lighting>Scene>Ambient Source).
The default value is for the ‘Ambient Source’ property to be set to ‘Skybox’. The Skybox in this case is the default procedural Skybox that - with default settings - contributes a blue tint to the Ambient Lighting of the scene. The other options for the ‘Ambient Source’ include a solid color, or a ‘Gradient’ which is a simple color ramp applied over the hemisphere.
Note that changing the color of the ambient source does not affect the visible Skybox, instead it only affects the color of lighting within the scene.