it means making a rifle better,smoother,more accurate,more consistent,more shots per fill (pcp's) or easier to cock (springers) it can involve polishing internal moving parts, different springs or fitting regulators. it can be a variety of prices depending on rifle type and how far you wish to take it! a v-mach kit (diy) is around £70 and for my prosport consisted of a spring, a guide,a top hat , lube and a new piston seal. various shops do tuning to various degrees at various prices! people will be along soon to say their favorites i am sure.