The Magna Carta, drafted in 1215, was the solution to a political crisis between King John and those opposed to tax policies and disagreement with the Pope. The medieval feudal system essentially established the king as the creator, distributor, and mediator of laws in the land. Likewise, the laws were unclear and could change often. English barons rose up against King John and recruited allies in France and Scotland who were loyal to the Pope, who had excommunicated King James. A civil war broke out and after much fighting, John met with the rebels at Runnymede and there they negotiated and signed the Magna Carta. John later protested that he was coerced into signing the document, which was partly true, as he had to concede some power to stay on the throne.
The document itself enshrined protections and liberties for the Church, towns, and the individual. The document, for example, protected individuals form having their property seized and gave them the right to justice in a court. The document also stated that no one was above the law, not even the king.
So the Magna Carta, in essence, can be understood as the first major step in limiting the power of the feudal system by enshrining in law protections of religion, property, and individual liberty.