weight mapping applies to polygons, points, and their movement. So if you bend a model's elbow or knee, the polys assigned with 100% weight won't move, 75% weight will move a little, 50% will move more, 25% weight will move a lot, 0% weight will move the most (or it might be the reverse of this, but you get the idea). Having polys weighted appropriately will make for much more natural looking joint bends and other more natural looking posing overall. If I remember this right. Don't take this as gospel til you hear from a higher or more knowledgable authority!