It is a cover to prevent a zabuton from getting dirty.
The zabuton is supposed to be used without using any covers but once it gets dirty, it is difficult to wash it at home. For this reason, using zabuton covers becomes increasingly common in recent days. The cotton material is commonly used for a zabuton cover. There are also linen zabuton covers or rush grass zabuton covers for a summer zabuton. A zabuton isn’t square and the length of the item is a little longer than the width, so be careful about the direction when you put the cover on the item.