Magna Carta is a really old document that was written in England a long, long time ago. It was written in 1215! That's around 800 years ago!
The people who wrote it were called "barons". They were really important and powerful guys who were friends with the king of England at the time, King John. But even though they were friends, they didn't always get along. So, the barons decided they needed some rules to follow so they wouldn't fight with the king anymore. They called these rules the Magna Carta.
The Magna Carta had lots of rules about how the king and the barons should treat each other. It said that the king couldn't just do whatever he wanted to the barons anymore. The barons had rights, too! The king had to ask permission from the barons before he could collect taxes or go to war. If the king broke the rules, the barons could say "NO!" and make him follow the Magna Carta.
Now, you might be wondering why the Magna Carta is so important. Well, it was one of the first times in history that people wrote down a set of rules for how a country should be run. It helped protect people's rights and freedoms, and it inspired other countries to make their own rules like the Magna Carta. Even today, people still talk about it and use it as an important example of protecting people's rights.