When you are born, you become a baby, but as you grow up, people start to see you in different ways based on the things you do, the things you like, and the things you are good at. These ways people see you are called identities, and they form an important part of who you are.
Now, imagine that you become very good at playing soccer. People might start to see you as a soccer player and expect that you will keep playing soccer or even become a professional soccer player someday. This identity of being a soccer player becomes a big part of how people see you and how you see yourself.
In sociology, this kind of identity is called a "master status." A master status is the identity that other people most strongly associate with you or that you most strongly associate with yourself. It can overshadow other identities you might have, like being a good student or a good friend.
So when people think of you, they might think of you first and foremost as a soccer player, and that identity could affect how they treat you or what they expect from you. You might feel pressure to keep playing soccer and to be good at it because it's become such an important part of how you are seen by others.
In short, a master status is a really important identity that people give you or that you give yourself, and it shapes how other people see you and how you see yourself.