What is subinfeudation?

The feudal system was based on permanent heritable land grants in exchange for services. At its origin these services were military services, so the crown (the ultimate landowner) granted large tracts of land to nobles and in exchange the nobles pledged to provide so many knights or footsoldiers in case of war. Those great nobles could grant part of that land to lesser nobles in exchange for a promise of lesser military support, and these in turn made smaller grants until at some point there was a grant to a person (called the freeholder) who actually farmed or lived on the land, and had to provide simple services such as a certain number of days' labour or a certain quantity of grain. These lesser relationships between one who granted land and one who provided services, nested within each other like Russian dolls, were called subinfeudation.