Enclosure was a process that changed the nature of landholding in England. It took place over a long period of time, starting most notably in the Tudor period of the 1500s.
Before enclosure spread, most land in England was worked communally. The land would typically belong to one person or institution, such as a lord or an abbey. Even so, it would be worked not as one single farm but rather as a multitude of small family "farms." I put the word "farm" in quotes because families did not own and work specific pieces of land. Rather, they worked different plots each year according to what they were assigned at the start of a year.
Enclosure changed this. It took away the peasants' right to work a certain amount of land on their own. Instead, the person who owned the land would enclose it, make it in to one plot, and often raise sheep on it. This put the peasants off the land and was a very serious problem for them.