The purpose of the Magna Carta was to guarantee land owners and English gentry that they would not be unfairly taxed. The complaints that lead to the Magna Carta were not dissimilar from those that prompted the American Revolution.
The Magna Carta, or "Great Charter," was signed at Runnymeade Field in 1215. Prior to that date, England owned land interests in France. Traditionally, English kings had sought the approval of English landowners and gentry prior to raising taxes to fund the defense of English territories abroad, as per the custom of the English feudal system. King John, however, thought himself too important to seek consultation with land owners and began raising taxes as he pleased. Landowners were further angered by the fact that John's military endeavors for which they were being taxed without consent failed, and England lost its land interests in France. Much like the American colonists, English landowners felt that it was not fair of the king to tax them without providing them with representation. So, they literally surrounded King John at Runnymeade and forced him to sign the Magna Carta. It is said that King John was so hated that no King of England can ever again have the name John.