Magna Carta is known as the first Constitution the world has ever seen, and because of this has huge importance to all democracies around the world.
The Magna Carta came to life in a context of absolute monarchy, at that time King John of England was pushed to sign it by a group of rebel barons. They were tired of having their rights taken arbitrarily.
It has a huge importance because this document established the principle that everyone, even the king, is subject to the law and everyone has to guarantee the rights of individuals to justice and right to a fair trial. Those are still the basis of modern democracies.