Subinfeudation was the feudal custom of vassals holding vassals of their own. In the multiple layers of feudal obligation in medieval Europe such subinfeudation was common, with lords owing fealty to a greater lord, and receiving fealty from lesser ones.
This is not to say the the situation was always -- or even often -- that simple. In many cases, a medieval lord would owe leal service to multiple overlords, and could find himself in a sticky situation -- in a worst case owing conflicting service to two overlords at odds with one another.
Another problematic aspect of subinfeudation was the obligation of the lord to the vassal, and whether or not that obligation extended to the vassals of his vassal, a debate which had serious implications in period.