It's a bandana. It wicks sweat an moisture and doubles as a hanky. It weighs 0.9 oz., which more than justifies it's usefulness. In really hot weather I use it to keep my neck out of the sun to avoid sunburn. I also find it useful to keep my camera wrapped to avoid scratches.
Handy little bugger for less than an ounce.
Available from REI for $3.