Drew, lapping is rubbing two pieces of metal together with an abrasive paste so that the two pieces mate better or to assure one piece is perfectly flat.
Rough bores in firearms can be "fire lapped" by coating a bullet with such a paste and then firing them through the firearm.
As to lapping the bolt on a rifle, I have never done it so I can not advise on the methods used. I would assume that minute amounts of the lapping compound (such is the name of the abrasive paste) would be applied to the locking lugs of the bolt, the bolt inserted in the rifle and then open and clost the bolt several times. The number of times would depend on the grit of the lapping compound and how fast you wanted to screw up the rifle if you went too far causing excessive head space.
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