"Fitting a barrel" is the process of taking an oversized barrel and machining it to prefectly fit the handgun it is going into. By reducing the tolerances/clearances between the frame/slide/barrel you are able to increase the mechnical accuray of the firearm. Some people also report a reduce in recoil but that is usually just a smoother, not smaller, recoil impulse due to the fact the firearm cycles smooth when the main parts aren't banging around against each other.