Nori, it could be a name that your gym has given to trainers who meet certain qualifications for which they've set the standard -- an internal title (like "Head Trainer"). Or it could be some kind of certification from one of the 150+ organizations that certify personal trainers (of which there are maybe 5 -7 reputable ones). It's not a certification that my organization uses and I've never come across a "Master Trainer" so I just don't know.
Why not ask him? If he has the title, he's probably proud of what it took to earn it.
And I'm so glad that you're having a good experience!!
