If you imagine a pitch black room. That's what effect you'll get when ambience level is set to 0. Now imagine a bright sunny day. That's what you'll get when light is at 100.
The ambience level decides how bright the general light of a room is. It's not like a light (like, a lightbulb) which you insert in a game with the editor - those lights are put in specific places. Ambient light is all around.
Try making a simple level. Play it with quick light mapping on. Then, whilst playing, press < and > (not left and right, but the smaller than/greater than) keys (you might need shift, not sure) - this will adjust the ambient lighting in the level. Don't put any lights in to see the full effect of what ambient light does.