The answer is that there’s no single answer, there are different
Here is one of the best definitions you can find by the website barber-schools.org :
"What makes a Master Barber License different from a Standard or Restricted Barber License? In most cases, you’ll find that experience, expertise and quality of service set the regular barbers apart from the master barbers. They have mastered most men’s hairstyles and hair services, and have a more proficient level of skill in the barbering arts and sciences. Also, standard barber technicians may not be trained, licensed and permitted to perform certain services, whereas a Master Barber is."
And at Dashstylists (mobile barbers and hairstylists) we wouldn't give a very different definition : generally speaking, it’s the level of experience and skill that distinguishes master barbers from standard barbers and the fact that a Master Barber masterises more techniques.
Once a person receives their standard barber license, they can begin to train as a master barber. Master barber courses are available at many barber schools. However, requirements for the master barber license vary from state to state. In some states, obtaining a master barber license simply requires a person to have been a licensed barber for a certain number of years, say 15 or 20. In other states, after just a few years of possessing a standard license, a barber may obtain master barber status by passing a state licensing exam. And in yet other states, a barber must complete a certain number of ongoing advanced training hours and/or complete an apprenticeship under another master barber.
Barbers cut and style the hair of their predominantly male client base. They are required to be licensed in all fifty states since the profession deals directly with public health. Master barber training includes all the traditional elements of barber training, including hair cutting, hair coloring, facial hair services, the use of styling tools, hygiene protocols and client relations, but also requires further specialized training in these areas. The master barber license is a valuable asset, as it generally recognizes more experienced barbers as having the ability to provide a higher level of service which, in turn, gives them the ability to command higher prices for their services. But what exactly sets a master barber license apart from a standard barber license?
If you are looking for a legal and precise answer to the above question, it’s best to contact your state’s board of barbering directly.