Okay kiddo, have you ever heard your parents ask you to do something, like clean your room or finish your homework, and when you don't do it, they give you a consequence? That's kind of what a disciplinarian does.
A disciplinarian is someone in charge who makes sure everyone follows the rules. They may be a boss, teacher, or parent. They want to help you learn what is right and wrong, but they also want to make sure you are behaving properly.
If you break the rules, a disciplinarian might give you a "punishment" or consequence to help you learn why it's important to follow the rules. For example, if you keep interrupting the teacher during class, the teacher might make you sit in the corner or take away your recess time so you can learn to listen and be patient.
It's kind of like how your mom or dad might take away your iPad if you don't do your chores because they want to teach you that there are consequences for not doing what you're supposed to do.
Overall, a disciplinarian has an important job of making sure everyone is doing what they are supposed to, and if they aren't they receive a consequence to help them learn why following the rules is important.