Even though an object in a scene is not directly lit it will still be visible. This is because light is reflected indirectly from nearby objects. A simple hack that is commonly used to model this indirect illumination is to use of an ambient light source.
The look of a scene with only Ambient Light Source is very flat
Ambient light is the illumination of an object caused by reflected light from
other surfaces. To calculate this exactly would be very complicated. A simple
model assumes ambient light is uniform in the environment.