A masterclass is a lesson given by someone famous and very skilled — at first, often a great musician — to students who are already good. Other people may sit and watch. The teacher does not start from the beginning. They work on small, high-level points. Though actually, people now use masterclass much more loosely: a cooking class, a writing workshop, even a paid video series may be called a masterclass. In advertisements it can just mean “a class with a star,” and it may not be as rare as the old music sense. In music schools, the older meaning is still the real one.