This is a continuous loop or pocket that is sewn into the bottom of a drop or other soft scenic piece. Steel pipe (most often 1/2" schedule 40, commonly known as bottom pipe) that is threaded on both ends and connected together via pipe couplers are fed into the pocket and is used to keep the drop tight and weighted down. This removes the wrinkles from the fabric and creates a smooth surface on the fabric.
Picture courtesy Sew What?
Picture courtesy Sew What?