That would be Water Soluble Stabilizer, one of the materials used in machine embroidery that can NOT be omitted if the item has no opening to remove it once complete.
As posted last week I am working on zipper pouches, several of them. The problem is the most appropriate hoop is much too big and would waste expensive WSS. So I came up with this solution
Using fabric I cut an opening an inch larger than the design and marked horizontal and vertical lines, these lines match the ones on my hoop. A few pins and I sewed the stabilizer in with large stitches.
With this method the surface was strong, taking the piece on and off the machine after each step adding materials as needed.
The last step prior to turning the pouch right side out is cutting off all the excess 1/4" all the way around. On the hoop WSS easily tears away removing all but the stitches, after doing 10 pouches the build up from sewing in the WSS was too much so I used a seam ripper on my template to make a couple breaks in the stitches and removed them. I was back in business to make more
Before this method the size WSS piece needed to cover the hoop and stay secure was 12" x 14" Now that I can sew it in, I cut the WSS 6" x 8" for this 3" x 4" zipper pouch. The savings transfers to the cost I need to charge on etsy. Win Win!
(All my pouches finish without exposed seams thanks to Embroidery Gardens digital pattern (EmbroideryGarden.com) are washable on cold and dry on a low setting. This removes the WSS and any spray used)