This document restated the Provisions of Oxford, but added additional changes to taxation and inheritance policies, and included a section on mortmain.
The term "mortmain" comes from Latin mortua manus ("dead hand"), and refers to the permanent ownership of property by a legal entity, such as a church owning property from which it gains revenue through rent. Because the church entity was not a living person, the land could be said to be held not by a living person but in a "dead hand." (We think that is the explanation of the term, but no one knows for certain.)
Land held by an individual could be passed on to another when the individual died. If the individual had no heirs, the Crown took possession and chose to whom it should go. Mortmain meant that a legal entity could posses land eternally. Since a diocese or parish did not die, land possessed by such an entity stayed in Church hands in perpetuity.
The Provisions of Westminster put limits on mortmain, requiring approval from secular authorities (king and council) for the Church to hold land in perpetuity. (More limits were created in later administrations, such as by Henry's son Edward I).
There was more to the Provisions than mortmain, of course. Some additional parts claim that an accusation of murder could not be brought when a death was caused by accident. Also, a person could not be fined for making an honest mistake ("I didn't know I was trespassing" for instance).
A few years after this, however, the Provisions of Oxford and their successor, the Provisions of Westminster, were overturned completely. This would set up the Second Barons' War. I'll tell you how they were overturned next time.
(By the way, you can read Modern English translations of the Provisions of Oxford and Westminster here.)
