>>48513121A Pokemon Master is exactly what it sounds like; a master Pokemon Trainer, someone more skilled than a Journeyman Trainer.
I believe it was in something like Electric Tale of Pikachu, that Misty and Brock are referred to as Pokemon Masters, so you can extrapolate from there that every Gym Leader is a Pokemon Master. It would do them one better to explain what the benefits of being a Gym Leader are, since almost all of them have secondary occupations and stuff, but they seem to have some form of authority, as well as commanding respect from everyone around them for their skills.
The anime keeps it deliberately vague because if it ever became a quantifiable goal, Ash's perennial youthtime journey would become inexcusable. But in respectable continuity, it simply refers to an expert.
