Degenerate: Lapping is the art of milling and/or sanding your <component name here; e.g. Heatsink, CPU, memory, bearing, etc> until it has a very flat, smooth surface, within certain tolerances. In our useage, we want to get the smoothest surface possible on our heatsinks, because the best heat transfer occurs between a metal to metal surface. In a perfect world, the Heatsink would be attached directly to the CPU die without any heatsink compound, because the two surfaces would mate perfectly and would require no compound to fill in the little crevices. (Also, in a perfect world, when those two surfaces would mate, they would literally lock together). The smoother the surfaces, the fewer imperfections present, the more metal on metal you can see, and the smaller quantities of HS compound required.
Hope this answers somewhat intelligently.
And, Jerboy, I had wondered about SoftScrub for the final polishing. All the polishing compounds I looked at @ pepboys looked like they left a "protectant" film, which I would think would defeat the purpose of the whole lapping process in the first place.
Haven't had time to do the lapping yet, but I have a project that I will practice with first, before I try it on my Taisol GCK760092. Got a lapping kit from crazypc with 400, 600, 800, 1000, 1200, 1500. And the HS I will practice on looks pretty concave against the CPU, so I think I'll get a good work out 🙂