Why is the Magna Carta important?

Although it was written just over 800 years ago, Magna Carta remains important today largely because it was the first statement of the idea of limited government in the modern world. 
Before Magna Carta (and, in many places, for a long time afterwards) governments were above the law.  Governments were controlled by monarchs who had absolute power.  There were no laws that monarchs had to obey.  A monarch and his or her government had no limits.  They could do anything that they wanted to do and there was nothing that could stop them. 
Magna Carta was, in a sense, the beginning of the end of this idea.  Among other things, Magna Carta set out a number of rules that the king of England agreed to obey.  Most famously, it established that no “free man” (though this was a small group at the time) could be punished for any crime unless they had been tried and convicted by a jury of their peers. 
Even after Magna Carta, monarchs had much more power than we would let our governments have today.  However, Magna Carta started the trend toward limited government.  It set out the idea that even the government has to obey laws.  This is a very powerful idea, one that has shaped the world in which we live today.
 
https://www.bl.uk/magna-carta/articles/magna-carta-an-introduction