To "slug" a barrel, is to push a soft lead slug through it and then measure the diameter of the slug afterwards. The advantage of this is that you know the exact inner diameter of your barrel at its narrowest point. During the process of pushing the slug through the barrel, you will also learn how many (if any) "tight spots" are within the barrel. A consistent barrel diameter makes a difference in accuracy, as it results in less deformation to the bullet during the process of exiting the barrel.
Ideally, the slug you use would be a cylinder roughly the size of your barrel's bore. Generally speaking, you will either have to make your own or start with a projectile which is around the right size. One approach to use in making your own, is to cut a fired case from the rifle you want to slug, and pour molten lead into the neck. Once you've removed the lead, you will have a cylinder slightly larger than your bore diameter.
ARTICLE INCOMPLETE - will hopefully get around to writing more later.
-- SeanNewton - 12 Mar 2008