It's what they sell in fly shops as 'body braid'. Plastic stuff woven into a tube. A bit of a metallic or pearlized shine to it.
Unlike what I know as 'mylar', it has no core. Used to be hard to find this stuff even at fly shops. All of a sudden, it's everywhere,and unlike the fly shop, it's inexpensive.
I use it for overbodies on foam poppers and for bellies on a rattling pike minnow I tie, a few other things.