lapping the rings means the gunsmith takes a piece of bar stock, let's say 30mm in diameter, spreads lapping compound (think really fine, liquid sand paper) and smooths out the rings. the reason for this is so the high spots on the rings don't pinch the scope. not pinching the scope means no scope ring marks and a better, more consitent contact area.
i had it done. i was talked into it buy a "gunsmith". guess what, the fucker charged me $75 (i know, i know, i got ripped off) and i still had ring marks.
i am not sold on ring lapping.