A more thorough explanation:

sacramentum fidelitatis (sak-rə-men-təm fi-del-ə-tay-tis). [Law Latin] Hist. The oath of fidelity, given by a vassal to a lord.

During the medieval period, a vassal would swear the sacramentum fidelitatis to their lord, pledging their loyalty and obedience in exchange for protection and land.

For example, a knight might swear the sacramentum fidelitatis to their lord, promising to serve and defend them in battle.

Another example would be a serf swearing the sacramentum fidelitatis to their lord, agreeing to work the land and pay taxes in exchange for the lord's protection.