On the 19 June 1215 at Runnymede King John signed the Magna Carta. (This means Great Charter.)

It was the first formal document stating that a King had to follow the laws of the land and it guaranteed the rights of individuals against the wishes of the King. This meant people couldn't be arrested, imprisoned of have their possessions taken away except by the judgement of his equals and/or the law of the land. This laid the way for trial by jury which means people are tried by their peers and guaranteed the civil rights of the individual.

The Magna Carta established the principle that the people of England, at this stage represented by the Barons, could limit the power of a King, if he was doing things that were not good for the country
(BBC - Pimary History)

Richard, the Lionheart, King of England had spent much of his reign outside England fighting wars in the Middle East and France. To pay for these he had taxed the English heavily. In 1199, Richard died and his brother, John Lackland became king.

John continued to fight wars in France but he kept losing battles. He needed more money so his government in England ruthlessly demanded more taxes from the nobility who were expected to pay tax if the King asked.

The Barons became very unhappy about John exploiting their loyalty and belief in his complete power. They rebelled and took over London and forced John to negotiate.