Varies. Usually removal of all burrs, light polishing of contact surfaces, modified or replacement springs, and putting everything in correct adjustment.
If the person doing the work doesn't know what he is doing, the result may be ruining instead of tuning. I have, alas, seen too many of the former case.
Tuning a S&W or Ruger isn't hard. Mind you, most of the time you smooth out the trigger pull. That's different from making a trigger pull lighter. Best way to learn is to take an NRA summer gunsmithing course and learn under the tutelage of a gunsmith. Working on Python type actions is a different matter altogether and requires a lot of skill & understanding of mechanics. Jim's advice is right in that you can really "butcher" a gun if you don't know what you're doing.