This is a document which can be drawn up between a couple who have chosen to live together. When living together you are not protected in the same way that you would be if you were married or had entered into a Civil Partnership.
It can spell out the terms of a relationship and often address financial issues. For example, it may deal with how property owned by either of you at the beginning of the relationship is to be treated and how the shares in any property accrued during the period you live together is to be held. It can also deal with how the property is to be divided in the unfortunate scenario that your relationship ends.