King John of England was forced by the rebellious barons, to place his seal on the Great Charter as it came to be called on June 15th, 1215, thus limiting the king's autocracy and power over the people.
The Magna Carta is highly significant because it creatied the very concept of human rights, the idea that "all men are equal". Its lasting iconic value as the foundation of so many world democracies lies in the power of an idea - a principle, which states that nobody, including the King, is above the law of the land.
King John of England
Magna Carta - The Great Charter 1215