England had many kings, some good, some not so good, and some really terrible. King John was a terrible king. There are stories about him saying things like he had put a peasant in prison so he could seize the man's horse and cart. King John was also a liar. He lied to get his own way. He was also very domineering. He expected to get everything that he wanted just because he was king.
Promises meant nothing King John. He would promise anything if there was money in it for him, but as soon as he saw an advantage for himself, he broke his word.
King John didn't even care for the church. He seized the monastery at Canterbury when the archbishop died. This made the new archbishop of Canterbury furious. His monastery had been taken away by the king.
The archbishop decided to do something about King John. The archbishop called all the nobles of England together and read them the law about what they could do. The nobles were awed to learn that they had the legal right to do something about the king's bad behavior. But they did not do anything, not quite yet.
Shortly after this, King John raised the taxes without asking the nobles. The nobles knew this was against the law of England. The nobles got angry and went to King John. The King promised he would never do that again since it was against the law. Now remember how King John keeps his promises. A small amount of time later, King John raised the taxes again. When two of his nobles refused to pay, King John had them arrested.
The rest of the nobles were angry. They put on their armor and gathered a huge force of nobles together at the town of Runnymede. Armed and ready, they brought the king there. The archbishop read aloud to the king from a parchment he held, a parchment called the Magna Carta, the Great Charter. This document stated the rights of the nobles and what they could do to the King if he violated those rights.
King John did not want to sign this paper. If he did, there was no way he would be able to go back on his word. But there was nothing else he could do. Without the nobles and their knights, he did not have anyone to protect him or the kingdom. So he signed.
The Magna Carta is just a list of the rights of the nobles, but it is a list of rights signed by the king. From that point on, the Kings and Queens of England could only do so much without the help of the nobles, and later on, without the help of the people themselves. It was not democracy, but it was a very important step in that direction.