Feudalism is the name given to the social and economic system that predominated in Medieval Europe. Under this system, the Crown granted extensive land holdings to nobles in exchange for military service. Wars were quite common throughout this period, so this was a very important responsibility indeed. Nobles were responsible for raising and financing their own troops, drawing them from among the peasants who worked on their land.
At the bottom end of the feudal system, peasants were obliged to live and work on their lords' land, in return for which they were given a share of whatever they produced, as well as military protection. This meant that peasants were tied down to the land where they were born and couldn't move to any other part of the country. For the average medieval peasant, the only time he ever got to see the world outside his village was when he was away fighting for his lord.
The feudal system placed extraordinary power in the hands of the nobles. In many cases nobles used that power to extract concessions from the Crown. If the monarch in question happened to be weak, they would quickly back down, worried what might happen if they stood up to their nobles. Even the strongest monarchs, however, knew they had to keep their nobles on their side, otherwise they'd be likely to rebel, able as they were to put a large, private army together at a moment's notice.