Muristus is the fancy word for when people build a wall to separate places or things. Remember when you and your friends were playing and you made a fort out of pillows and blankets to keep your little brother out? That's a little bit like what muristus means.
Only instead of pillows and blankets, people use bricks or stones or concrete blocks to make a wall. And instead of brothers, the wall is used to separate bigger things like countries or cities or even rooms in a house.
Sometimes people make muristus to keep things safe, like building a fence around a baseball field to keep baseballs from flying off the field and hurting people. Other times people make muristus to keep things apart, like building a wall between two countries so people can't easily cross over from one to the other.
Muristus is just another way people use to keep things organized and safe.