The tag says
Liner, wet weather poncho. NSN 8405-00-889-3683
but nobody calls it that. It's a woobie. It's called that because without one, you woobie pretty cold and lonely.
It is probably the most highly regarded and useful piece of kit ever conceived by the US military. It's a thin, camouflage print nylon quilt full of synthetic batting, and was originally designed to be integrated into the GI poncho as a temperate weather liner via a horrible system of strings and eyelets. It turns out the poncho wasn't worth a damn so they quit making it, but the woobie is still around with absolutely no changes other than a color update to match the new uniform patterns. You can't improve what's already perfect.
It's soft, warm, dries unbelievably quickly, and can be stuffed into places that don't seem possible to fit a handkerchief, let alone a 60x80 inch quilt.
You can wrap yourself in it as a blanket, roll it up with a boot blousing strap to make a pillow, turn it into a privacy curtain for your bunk, dry off with it, line your sleeping bag with it, make a sun shield out of it, insulate a drafty doorframe with it, use it as a laundry bag, a partition in your duffel bag, drench it with water and use it to cool off, and on your last day in country, you can tie it around your neck like a cape and scream around wearing nothing else but a pair of boots and a bunch of airplane noises.
It's everything a towel is to a galactic hitchhiker and then some.
Mine has been patched and repatched; defaced with Marker, permanent, wide tip NSN 7520-00-L05-4035; scuffed, stuffed, rolled, and dragged all over the nastiest parts of the planet, and it has never failed to do or be exactly what I needed any time I pulled it out.
Have you ever seen a grown man drag a blankie around? Have you ever seen two grown men get in a fistfight over a blankie?
I've been one and seen the other. Someone's woobie got misplaced and there was a great deal of commotion until a bystander found it under a pile of body armor.