Lapping, is a process in which you sand your processor or heatsink until the surface is flat as it can be.  
Usually you'll lay down a piece of sand paper on a flat surface, like glass.  Wet the sand paper a bit and hold your heatsink flat and sand in circular motions.  Keep sanding until the surface is totally flat.  These days hardly anyone ever lap the cpu anymore unless you have an older celeron, because it's extremely dangerous to lap the cpu now because of the new packaging.