This postcard shows the Peterborough Cathedral Cloisters. You can go to it via a huge door from the Cathedral's right side. If you wish to go directly to it there is a pathway to the right of the Cathedral in which you can go to.
According to Wikipedia, a cloister (from Latin claustrum, "enclosure") is a rectangular open space surrounded by covered walks or open galleries, with open arcades on the inner side, running along the walls of buildings and forming a quadrangle or garth. The attachment of a cloister to a cathedral or church, commonly against a warm southern flank, usually indicates that it is (or once was) part of a monastic foundation, "forming a continuous and solid architectural barrier... that effectively separates the world of the monks from that of the serfs and workmen, whose lives and works went on outside and around the cloister."