Before the Magna Carta, feudalism was how people ran the kingdom. Feudalism was a sort of social pyramid that had classes like peasants and nobles.
The king could do whatever they wanted, for they ruled with divine right. They could imprison anyone for no reason or take peoples stuff as they wished. King John took advantage of this and abused his power.
The nobles weren't happy with this. They wanted more rights and not let the king do anything.
So they made the Magna Carta. This took away the king's rights and gave the nobles more.
They brought the Magna Carta to King John. He refused to sign it, but was threatened to be killed and forced to sign.
Almost all of England liked the idea, but at first only nobles had more power. However this slowly applied to more people, as the Bubonic Plague wiped out most of England, making survivors more valuable.