I bought this Rug Backing at Walmart, in their crafts section, about two years ago. Other people in London, Ontario, have told me that they can't find it there anymore. The name at the top of the canister says M.C.G. Textiles, Rug Backing by Saf-T-Bak and it is available at Amazon.
Here's what the product description says:
Saf-T-Bak will make your rugs highly resistant to sliding. Saf-T-Bak will increase the life of your rugs. It stays on after repeated washings: anchors loops and fibers thus preventing unraveling and tuft loss. It helps your rugs retain their shape and protects the base from moth damage. Saf-T-Bak elimintes wrinkles, adding bulk and texture. A small amount of Saf-T-Bak applied to frayed edges prevents further fraying.
It's a "water emulsion of synthetic rubber". I've washed my slippers and they came out just fine. They don't slip when I walk on wet floors. The only problem I can see is that it's white, which shows the dirt. Other than that, I highly recommend it.