Rulers During the Feudal System
At the very top of the feudal society were the monarchs. They were expected to keep order in their kingdom and provide for their vassals. In exchange for providing for these vassals, the king or queen would rely on them to provide knights and soldiers for their armies, and take care of the land he would give them.
Sometimes, when a monarch would die and there wasn't a family member or close relative to take on the role of a king or queen, there would be confusion as to who would be the next ruler. One example of this problem was when William, the powerful Duke of Normandy, believed he had the right to the English throne. However, the English crowned his cousin. William and his army then invaded England, meanwhile defeating his cousin. His triumph earned his title, "William the Conqueror".
To conclude, the monarch was a huge part of the feudal system. Without this role, the system couldn't have brought stability to Europe in the middle ages.