A fief was a piece of land granted by a lord to a vassal in the feudal system of medieval Europe. In exchange for the land, the vassal promised to provide military service and loyalty to the lord. This arrangement created a bond of mutual obligation between the two parties. Fiefs were essential for the organization of society during this period, as they provided the economic basis for the lord's power and the vassal's livelihood. The system helped maintain order and stability, allowing for the management of land and resources in a time when centralized government was limited.