Okay, imagine you're playing a game of pretend make-believe with your friends. You might decide that one person is the "captain" and they get to make big decisions for the game and tell everyone what to do.
In real life, there are some jobs where someone is like the captain of a group of people. One of those jobs is called a captain-major.
Let's say you're in a group of people who work together to keep a city safe. The captain-major is a person who is in charge of making big decisions about how to keep the city safe, and they tell all the other people what needs to be done.
It's kind of like if you were playing a game and the captain-major is the person in charge of the game, except this is real life and people's safety is on the line. So the captain-major is a really important job, and the person who does it has to be very responsible and make good choices.