Yep. What the other commenters said. Silk sword bag for storage or transporting a Katana.

You can get these on Amazon, but they usually come with Katanas in the box when you buy a sword.

On Amazon:

https://www.amazon.com/dp/B072Q988ZH/ref=cm_sw_r_api_i_QCQJKM4TQK8G0JNTD39M_0?pldnSite=1

I’ve bought several from Amazon because sometimes katanas don’t come with one, or it’s a thin nylon black one that’s ugly that comes with a sword.

They are inexpensive for some pretty impressive looking bags.