He sends down water from the skies and each channel starts flowing according to its measure, and then the torrent bears a swelling foam – like the scum which appears from metal which are melted in the furnace for making ornaments and utensils. This is how Allah compares truth to falsehood. As for the scum, it is thrown away, being worthless, but that which is useful for the mankind remains behind on the earth. In this way Allah cites examples to make His message clear.
Meaning:-
Torrent: a strong and fast-moving stream of water or other liquid.
Scum: a layer of unpleasant or unwanted material that has formed on the top of a liquid or body of water
Cites: to mention in support, proof, or confirmation;