I learned about normals quite a long time ago when i was still fiddeling with vector-graphics on a Sinclair-Spectrum computer. Normals is the direction in wich a surface is pointing seen from the observers point of view. If the surface is pointing more than 90 degrees away from the observer than it is being regarded as not visible. It is not completely the same, but it should give you an idea

Robert van der Veeke Basugasubasubasu Basugasubakuhaku Gasubakuhakuhaku!! "Better is the enemy of good enough." Dr. Mikoyan of the Mikoyan Gurevich Design Bureau.