Horsehair braid is a stiff, bias woven mesh used to provide structure and body to special occasion and wedding dresses, as well as costumes. Available in multiple colors.
Horsehair braid acts as both a facing and a hem stabilizer. It's used to add structure to hemlines.
Horsehair braid used to actually be made of horse's hair, but now it's made of nylon or polyester. You can buy it in various widths. A wider width will give you more body to your hem, and a narrower one will be more subtle.