The Magna Carta today is seen as a major statement of the idea of limited government. At the time when it was written, it was not quite such a grand document. Instead, it was one that simply shifted the power between the monarch and the lesser nobility.
What the Magna Carta did at the time was to redefine and reemphasize the feudal relationship between the king and his vassals. It simply made clear that the king had responsibilities to his vassals and could not just rule as an absolute monarch. This was already an idea that was firmly rooted in feudal tradition, but the Magna Carta put it formally on paper.
At the time, then, the main importance of the Magna Carta was that it decreased the power of the king and gave more weight to the rights of the lesser nobility.