Lapping a barrel involves using some sort of MILDLY abrasive substance that is run through the barrel to remove any burrs, or other irregularities in the bore. This may be done to simply "clean up" the bore and make it really smooth, or it may, in more serious cases, actually be done - with a more abrasive compound - to remove some metal and make the bore slightly larger.

EDIT: Actually, you should check out the post titled, "CAYUGAD" of 1/12/2006. It gives all the detailed info, as well as a more precise definition of these terms.

IM jaybe