The lord of the land (the feudal lord) grants land (a fief) on his vassals (the people). In exchange for the fief, the vassal must provide military service to his lord. This military service provided to the lord has dual purpose. First, they serve there lord via military service. second, the military servicemen defends themselves and the people living inside the domain of their lord, thus protecting the interests and the properties and the vassals of the lord of the land (the feudal lord). It is this system between the lord and the vassal, and the fief forms what we know as feudalism.