The word dross is a noun that means “something that is base, trivial, or inferior”, “an impurity”, or more specifically, “the scum that forms on the surface of molten metal”.
It’s related to the word dregs, a noun referring to the sediment that forms in some liquids. Both dregs and dross are used figuratively to describe things that nobody wants.
Similar to dross (and just as yucky-sounding), the word slag refers to a byproduct of smelting metal. What exactly is the difference between dross and slag? Wikipedia explains that “dross, as a solid, is distinguished from slag, which is a liquid.” So there you have it.
Solid or liquid, the words dregs, dross, and slag all sound exactly like what they are.