Magna Carta was a document that Bad King John
signed sealed in 1215, it has been described as the basis of constitutional law and an important part of the foundations of the liberties of all English speaking peoples.
Magna Carta is Latin for great charter, at that time Latin was used for important official documents and English was only spoken by the peasants.
John didn't want to set his seal on the document, but his Barons, the wealthy landowners who had much of the power in feudal England forced him into it. They didn't particularly want him to seal it either as they would rather have got rid of him, but for lack of a credible replacement King they decided to limit the power of the one they had. They certainly didn't want to establish legal rights for the common man - most of whom were in effect their property as Serfdom is close to Slavery.
Most of the provisions of Magna Carta have long been superseded, but more importantly they have been broadened from only protecting a wealthy minority of "freeborn" men from arbitrary royal power to protecting all of us. But when we say that people have a right to a fair trial with innocence or guilt to be determined by a jury of their peers, we can pretend to invoke Magna Carta.
Of course real radicals like Tom Paine long ago rejected the idea of basing our liberties on a document by a King, he said that human rights were inalienable. But Magna Carta is a great argument to use with Conservatives because its old, and its Royal.