Answer:
Common land was to be used by the common people, with restrictions based on custom and law, and subject to governance by people who were responsible for it.
Common land on a manorial estate was usually used for pasture. Some of it was also used to provide firewood.
There were circumstances under which a man could build a home on common land, but only a small one. One law I have seen on this says a man could build a cottage on common land if the construction could be finished within the course of a single day.