This is a pre-printed piece of WHITE cotton fabric with a traditional Japanese pattern of waves called Seigaiha. It's very handy for learning sashiko stitching. Sashiko is a traditional Japanese style of quilting that is often used for decoration.
The fabric measures 13.25" x 13.25" (34 cm x 34 cm) folded in half and is printed with dye that will wash out. Sashiko needles, thread and thimbles are also available in my shop.