It is just a term for the way the rear apeture looks when you have a correct sight picture. When you focus on the front sight post, the rear sight will get kinda fuzzy and won't draw the attention of your eye away from the front sight. With the MAD, I've heard that since there is so much metal around the actual apeture, your eye has a harder time getting a proper sight picture. I've never used one myself so I can't really say what it is like for sure.