"Death From Above"
It means good! It's the measurment between the face of the bolt to a spot on the shoulder of the chamber. There are two different versions of head space. Without going into detail, one is called SAMMI and the other is the Military or NATO spec. What does that translate to you? You should be good to go shooting both mil surp ammo and commercial ammo. There is a thread you should read and search for written buy Gus Fisher called a little but of everything to do with head space. It will spell out the differences in great detail.