Lapping is a smoothing or polishing of a surface. The purpose people do this do a CPU or heatsink is this: CPU - reduce the amount of material between the core of the processor and the top surface, also try to increase the trueness of the surface and eliminate more surface irregularities. Heatsink - Increase trueness of the base of the heatsink surface, and reduce surface irregularities. What this does is increase the heat transfer from the CPU to the heatsink. If the surfaces of the to CPU and heatsink are true, flat, with as few pits, scratches, etc... on the surfaces this will allow better contact between them. Now the main purpose of heatsink compound, grease, is to fill in these irregular surfaces and assist in heat transfer. If the surfaces are better to begin with from the lapping that the transfer will be even better. I really feel the most benefit comes from reducing the amount of material on the top of the CPU, thus getting the actual heatsink closer to the heat source and allowing it to perform its function better.