A dust ruffle covers the space between a bed and the floor. It's usually a ruffled strip of cloth attached to a rectangular cloth that goes between the mattress and the box springs or base. In some cases an elastic band or clips are used to keep the ruffle in place rather than an anchoring rectangle of cloth. The ruffle then hangs down from below the mattress to the floor covering the base, legs and blocking the space under the bed. Because it blocks the view of that space, it can be used as storage. Some storage boxes are specially designed to fit in that space.
Also known as a dust ruffle or valance.