The Magna Carta (which is Latin for Great Paper, or Great Charter) was so important because it effectively forced the reigning monarch to grant his English subjects rights. It laid the foundations for Parliament and constitutional governments.
It was drawn up by the Archbishop Stephen Langton and some of the most powerful land barons of England. It was agreed to by King John in 1215, shortly before Pope Innocent III nullified it and triggered civil war in England. Nevertheless, it was reissued a number of times after the death of King John.
Among the principles of the Magna Carta were that no free man may be imprisoned, banished or otherwise destroyed without the due process of law, and that this due process of law must apply to everyone, free from the interference of bribery.
This document paved the way for the United States Constitution and Bill of Rights.