Lapping the bolt is generally done by applying a sort of abrasive compound between the bolt lugs and where they bear on the receiver. This is done to achieve adequate contact between the two surfaces. Fring pressure pushes the bolt against the receiver at this point, so insufficient contact can cause cracking and failure. Of course, this must all be done while maintaining proper headspace (or it MAY be done to achieve proper headspace if it's a LITTLE tight). One should go easy, though, as you don't want to go through the surface hardening on the bolt or receiver, or you could re-heattreat the parts. A bit beyond most hobbyists.
As 30Cal said, shimming the gas system is accomplished by inserting the appropriate thickness shims between the barrel shoulder and front band to achieve the two goals he stated: proper alignment of the gas ports and tight lockup to reduce movement of stuff when firing the rifle. If you have gas port alignment problems, a proper shimming job can correct them. If you don't, an improper shimming job can produce them. Sufficient misalignment WILL cause reliability problems.