Imagine you are learning how to draw pictures. You might take a class to learn how to draw better. A master class is a special kind of class where someone who is really, really good at something teaches other people how to do it too.
Let's say a famous artist who can draw really amazing pictures decides to teach a master class. They will teach you special techniques and tricks that they use to make great art. You can watch how they do it and try to do it yourself.
In a master class, you get to learn from someone who is like a master chef making the most delicious food or a master musician playing the most beautiful music. They are really good at what they do, and they want to share their knowledge with you so you can improve too.