Matthew Paris, the great medieval chronicler did not think very highly of King John, the English King whose bad behaviour was the catalyst for Magna Carta :

The new exhibition at The British Library, Magna Carta: Law, Liberty, Legacy takes us through the various ways John was a bad King before following the legacy of this document over the years, from the English Civil War, to the American War of Independence and finally our own troubled times.

If you like medieval manuscripts, the first two rooms are worth waiting for. The actual documents themselves are hard to decipher, impossible if you don't know your latin, but being so close to them is much better than any digital reproduction. On top of the language difficulties however, the writing is often tiny. They must have had good eye sight in the 13th Century.

If you want a short introduction to Magna Carta and its history, the British Library has a good introduction. The "Great Charter" is not unique, and was not the first such document between a monarch and his subjects, but its fame rests on the way it was held up and used in the centuries afterwards. It was important in the transition from the "Kings Law" to the "Common Law", codifying some of the things we take for granted today, such as trial by a jury of peers and no arbitary detention.

Magna Carta is still a rallying cry, and that's a good thing.