Dust is the fine, powdery substance that settles on surfaces over time or is blown around outdoors. A junk shop might be full of beautiful old furniture and books, all covered with dust.
There's the dust you clean with a feather duster, and then there's the verb "to dust." You can dust your bookshelves, cleaning the dust from them, but you can also dust the surface of a cake with powdered sugar, covering it lightly. Informally, when something "gathers dust," it goes unused, and when "the dust settles," a situation calms down.