the.com/bed skirt

means A decorative fabric panel that hangs from the mattress to the floor, hiding the box spring, bed frame legs, and whatever storage boxes or dust bunnies live in that shadowy under-bed zone.

from Straightforward compound of 'bed' plus 'skirt,' using skirt in its old sense of a hanging piece of fabric that covers the lower part of something — the same logic behind a 'skirting board' or a tablecloth's drop. It's sometimes called a 'dust ruffle' or 'valance' (the latter borrowed from the fabric pelmets hung over curtain rods), reflecting its practical origin as a way to keep dust and drafts off the space beneath a bed, long before it became decorative real estate for hiding clutter.