Wikipedia tells us that the wa means Japanese, and shi means paper. Washi was traditionally used to make shoji screens, paper lanterns and some items of clothing. Its long fibers makes it very strong, and it is often used in origami.
Here’s Keith Houston’s account of learning to make a washi paper at Chrissie Heughan’s studio in Edinburgh from his Shady Characters blog. She uses kobo bark mixed with some linen fibers.
In this post from Open Culture you’ll find a somewhat lyrical impression of making washi. However I think the video below, from the Sekishu-Banshi Craftsmen’s Association, gives a clearer impression of what’s going on.
One surprising feature is the way they stack the wet sheets and dry them off in one pile without absorbent separators. You’d think the individual sheets would stick together, but they obviously don’t. I guess the bark fibers are long and tough enough to keep their formation.