Ambient light comes from somewhere, either a window or door or skylight, it can even come from a lamp. What happens to make it ambient and not directional per say, is that the light bounces all over the place, filling in all the shadows and diffusing the light by scattering it everywhere. So, even though the light is ambient, it can still be brighter on some planes than others. The planes that are turned away from this light will be darker, but what keeps them from being dark shadows, is all the light bouncing around filling in those planes with light. I hope I explained that in a understandable way and I hope that helps :)