The ambient light illuminates the object's surface equally in all directions on which it's applied. All faces receive an equal amount of light; therefore, no change in color can be observed on the complete object. Ambient is basically a mixture of two components: the color intensity of light and material.
An ambient light plays a vital role in Phong and Gouraud shadings; the diffuse and specular color components of these shadings are computed by using the direction of the light that falls on the object. Therefore, an object may receive less or no light on its side or back faces depending on the direction of light on the object. In such cases, the faces may appear invisible because of the black light that is generated; choosing the correct ambient light and material color will help in making these darkened faces visible.