The light you see in dark shadowed areas comes from diffuse reflection off of other objects. This light cannot be directly modeled using ray-tracing. However we can use a trick called ambient lighting to simulate the light inside a shadowed area.
Ambient light is light that is scattered everywhere in the room.
It bounces all over the place and manages to light objects up a bit even where no light is directly shining. Computing real ambient light would take far too much time, so we simulate ambient light by adding a small amount of white light to each texture whether or not a light is actually shining on that texture. This means that the portions of a shape that are completely in shadow will still have a little bit of their surface color. It's almost as if the texture glows, though the ambient light in a texture only affects the shape it is used on. The ambient keyword controls the amount of ambient light.