The Magna Carta (latin for "the great charter") was written and signed in 1215. The document is considered to be one of the building blocks upon which "constitutional law", as we know it, is based.
The Magna Carta is, essentially, a legal document that was forced upon King John of England by his barons and required him to renounce certain rights, respect certain legal proceduresand accept that the will of the king could be bound by the law. It explicitly protected certain rights of the King's subjects - whether free or unfree - most notably the right of Habeas Corpus, meaning that they had rights against unlawful imprisonment.
For more details, check this Wikipedia entry.