Congo Hair is a polypropylene yarn, similar in composition to products like EP Fibers, Widow's Web, Gator Hair, ALM Sea Fibers, etc but a lot cheaper in price. Tends to shed water, making it useful in a variety of tying applications - streamers, like Alan said, but also wings on dry flies (like Trudes, parachutes, Wulffs, hoppers, cripples), shucks on emergers, wing pads on emergers, wrapped bodies, ribbing. Lots and lots of uses.

Regards,
Scott