This decorated box was used to hold the corporal. This is the linen cloth used by the priest with the chalice and paten during the celebration of Christian mass.
The velvet was made during the period 1400-1500, but the box may have been constructed later using a piece of velvet which had come from another source. High-quality textiles were often donated to the church to be made into vestments and ecclesiastical furnishings. It was an acceptable way of recycling expensive fabrics with patterns that were no longer fashionable.