The Magna Carta, signed in 1215, is significant as it established the principle that everyone, including the king, is subject to the law. This document was a response to King John's tyrannical rule and grievances from his barons. It introduced fundamental rights such as due process and the right to a fair trial, laying the groundwork for modern constitutional law and liberties. The Magna Carta has since become a symbol of the struggle for individual rights and limitations on government power, influencing legal systems and democratic principles across the world.