Bob, normals aint that easy to put into words, but imagine a wall you have wallpapered, the wallpaper pattern is facing into the room. If for what ever reason the normals have been flipped then the wallper pattern would be stuck to the wall and all you would see is the back of the paper.

In Reality/Lux, what you would get is a black space, because the light would go straight through it. Here's a couple pics to show what I mean - the first shows a cube I made, please note all the lights. I've flipped one of the normals around, it is there but the normal is facing inwards.