You can find quite a lot of information about it in the Gunsmithing forum down this board a little bit. The basics are a lead bullet impregnated with abrasive is loaded into a shell and fired through your gun polishing the barrel. It must be done carefully and to the instructions that come with the fire lapping kit. It has supporters but also those who are against its use, saying it wears out the throat on a barrel un-necessarily. I have never used it, not having a need.
I am sure you will get quite a few responses on this, most likely on both sides.