In overclocking, this is a method to flatten the surface of your heatsink which interfaces with the processor. This is really getting close to voodoo, but some people have had good results. I've never done this, so this is just interpretation of many, many guides:

Get something that's almost certainly really close to flat™. A small sheet of glass is pretty popular. Also aquire a plethora of sandpaper with varying grit. What you want to do is tape the heaviest piece of sandpaper to the glass, wet the sandpaper (if it's that kind), and rub the heatsink against it. According to a guy who grinds homemade lenses for telescopes, the figure 8 pattern espoused by many lapping guides will actually exacerbate the natural convexity that will develop on your heatsink (the edges of the sink wear faster than the middle, so given a very large ammount of time you'd end up with a hemisphere instead of a flat surface). Work your way through the sandpaper, decreasing grit, until you've got a mirror shine on the sink. Apply thermal grease, stick it on, and smile at the 1 degree difference between the non-lapped heatsink -- and all it took was hours of mindless tedium!