"Master instructor" is a term that is becoming quite commonplace in Pilates teacher advertising. When I think of a master teacher, I think of a wise old Zen master, or at least someone who has many, many years of being an expert in their field. The Mirriam-Webster dictionary makes it a little easier to be a master, citing definitions such as: "an artist, performer, or player of consummate skill" and "a worker or artisan qualified to teach apprentices." One would hope that a Pilates instructor calling him or herself a master teacher would exhibit these qualities, but there is no guarantee.
At this time in the Pilates world, "master teacher" is popular a marketing term. It may very well refer to someone with master level experience, but there is no standardized master teacher certification program* or credentialing body overseeing the use of the term. That doesn't mean there aren't many master level Pilates teachers -- there are. For example, most people agree that the Pilates Elders are master teachers, and there are many others. But the bottom line is that excellent Pilates teachers may or may not call themselves master teachers, and so-so teachers may market themselves as a masters.
There are also a number of advanced Pilates teacher trainings that use the term master as part of their descriptions. These trainings usually require participants to have already graduated from a comprehensive Pilates instructor training program. Then again, there are plenty of specialized post graduate programs that don't happen to use the term master. If you are interested in an instructor with advanced education it is still best to ask directly about the extent of their studies.
It is not necessary to take classes from a "master teacher" to get quality Pilates instruction. There are many fine Pilates instructors who can take you a long way in your practice before you need a master teacher. If you are new to Pilates, my advice is to review the tips for finding a good instructor, familiarize yourself with what Pilates certification means and, if possible, talk to as many people as you can about their experiences with different instructors. Once you find a class you want to try, give it a few tries -- you will know if you are in the presence of a master.
For a deeper look at Pilates classes, instructors and certification, read these articles: