A huge broad funnel-shaped hopper-like device that's designed to fit down over-top of a basketball hoop, and to catch virtually all the shots so that they'll go through the hoop.
A standard hoop-skirt is made from heavy canvas or other thick easily-visible material, and is just a novelty/gag item that's intended to merely cause observers to snicker and roll their eyes as they watch the players score a hoop almost every time (similar to when a backyard jokester uses lengths of stove-pipe for guiding the ball when playing croquet). "Specialty" or "stealth" models of hoop-skirt are available, too, though --- they are made from ultra-transparent and non-reflective material in order to be virtually undetectable, and can be used to help "level the playing field" during actual team-competition games; they allow the less-capable/practiced side to increase their chances of beating --- or at least scoring a lot closer to --- their better-skilled opponents.