Okay kiddo, so there's this thing called the Anglo-Scottish border which is like a line that separates England and Scotland.
Back in the olden days (like a really really long time ago), people used to have wars and fights over who gets to control the land on the border. It was kind of like when you and your siblings fight over who gets to play with a toy.
To help stop the fighting, the government made a law called the March Law. This law said that people who lived near the border had to be responsible for keeping it peaceful.
So, if someone broke the law and caused trouble on the border, the nearby people had to work together to catch the person and bring them to justice. They had to do this even if the person who broke the law was their friend or family member.
The March Law helped to stop a lot of the fighting and made the border a more peaceful place.