This is a medieval burse, or Corporas case. The burse is a piece of liturgical equipment, consisting of two hinged pieces of stiffened fabric. They were used in the Middle Ages to store and transport a 'pair' of linens corporals, the two cloths used at the altar during the Mass. One of these corporals was placed under the chalice and on it the body of Christ was consecrated, hence the name; the other was folded to cover the top of the chalice.
The Wymondham burse is a rare survival. Worked in silk thread, it is primarily decorated with a 'tree of life' pattern and this dates the piece to the end of the thirteenth or beginning of the fourteenth century. This is accompanied by a series of shields of arms, which have been identified as those of the Warrenne, Say, Molintune, Gurney and Leuknor family. A group of families all connected in turn to the powerful Bigod family, Earls of Norfolk. The piece on the abbey website suggests that the Burse might have been an apprentice piece, I doubt it. The work is of the highest quality and its continued use and repair over a two hundred year period suggests that it was part of a important and treasured set of vestments. The burse may originally have been part of a set of vestments given by the Bigods to the important Benedictine Abbey of Wymondham.
The vegetable dyes used in the silkwork have faded over time and it is suggested that the reconstruction above is close to the original colour scheme. How this object survived the ravages of the Dissolution of the Monasteries and the Reformation is anybody's guess. One suggestion is that it fell to the bottom of a chest full of books and documents, whatever the the truth, it surfaced again in the eighteenth century.