Lapping the scope rings is where they are trued up while on the base with a device so they are more or less perfectly concentric with each other so that the scope tube sits in them without any uneven torque or stress when mounted on the scope base and screws tightened. Scope base/scope ring/firearm combinations may not be in perfect alignment due to manufacturing tolerances, and sometimes by design. Some scope rings have adjustments for side to side tolerances such as the Millett brand. I remember some rings would change their alignment just by reversing them; and it stated so in the mounting instructions.

http://www.cabelas.com/tools-accessories-wheeler-professional-scope-mounting-kits.shtml