The ceremony in which men became vassals to their lord was called: homage.
Homage is a feudal ceremony in which men acknowledges themselves to become vassals to their lord. A vassal in feudal society is an individual that is considered as having a mutual commitment to a feudal lord or monarch. A vassal vows fidelity and homage to his lord. Part of the mutual commitment is usually military support by knights to receive specific benefits such as keeping the land of the lord as a tenant or fief.