What is meant by "froth" or "foam" in Quran aya 13:17?

What is meant by the following aya from > transliteration.org : > 'Anzala Mina As-Samā'i Mā'an Fasālat 'Awdiyatun Biqadarihā Fāĥtamala As-Saylu **Zabadāan** Rābīāan ۚ Wa Mimmā Yūqidūna \Alayhi Fī An-Nāri Abtighā'a Ĥilyatin 'Aw Matā\in **Zabadun** Mithluhu ۚ Kadhālika Yađribu Allāhu Al-Ĥaqqa Wa Al-Bāţila ۚFa'ammā **Az-Zabadu** Fayadh habu Jufā'an ۖWa 'Ammā Mā Yanfa\`u An-Nāsa Fayamkuthu Fī Al-'Arđi ۚ Kadhālika Yađribu Allāhu Al-'Amthāla > He sends down water from the sky, and riverbeds flow according to their capacity. The current carries swelling froth. And from what they heat in fire of ornaments or utensils comes **a similar froth**. Thus God exemplifies truth and falsehood. As for the **froth, it is swept away**, but what benefits the people **remains in the ground**. Thus God presents the analogies. 013-017 > أَ‌ن‍‍زَلَ مِنَ ‌ال‍‍سَّم‍‍َ‍ا‌ءِ‌ م‍‍َ‍ا‌ء‌‌ ً‌ فَسَالَتْ ‌أَ‌وْ‌دِيَة‌‍ٌ‌ بِ‍‍قَ‍‍دَ‌رِهَا‌ فَاحْتَمَلَ ‌ال‍‍سَّ‍‍يْ‍‍لُ ‌زَبَد‌ا‌ ً‌ ‌‍رَ‌اب‍‍ِ‍ي‍‍ا‌ ً‌ ۚ‌وَمِ‍‍مَّ‍‍ا‌ يُوقِ‍‍د‍ُ‍‌ونَ عَلَ‍‍يْ‍‍هِ فِي ‌ال‍‍‍نّ‍‍‍َ‍ا‌ر‍ِ‍‌ ‌ابْ‍‍تِ‍‍غَ‍‍‍ا‌ءَ‌ حِلْيَةٍ ‌أَ‌وْ‌ مَت‍‍َ‍اع‌‍ٍ‌ ‌زَبَد‌ٌ‌ مِثْلُ‍‍هُ ۚ كَذَلِكَ يَ‍‍ضْ‍‍‍رِبُ‌اللَّ‍‍هُ ‌الْحَ‍‍قَّ ‌وَ‌الْبَاطِ‍‍لَ ۚ فَأَمَّ‍‍ا‌ ‌ال‍‍زَّبَدُ‌ فَيَذْهَبُ جُف‍‍َ‍ا‌ء‌ ً‌ ۖ ‌وَ‌أَمَّ‍‍ا‌ مَا‌ يَ‍‌‍ن‍‍فَعُ ‌ال‍‍‍نّ‍‍‍َ‍اسَ فَيَمْكُثُ فِي ‌الأَ‌رْ‍ضِ ۚ كَذَلِكَ يَ‍‍ضْ‍‍‍رِبُ‌اللَّ‍‍هُ ‌الأَمْثَالَ What is this "similar froth"; "zabadun"; that is being referred to? The first half of the verse I sort of understood to mean the slag oxides that may occur when smelting metal from ore, but then in second to last sentence it says "the froth is swept away, but what benefits people remains in the ground", possibly eluding to the process of sedimentary mineral formation. I realize that there is a much more important spiritual meaning of this aya. Information on the events and context of this aya's revelation would be deeply appreciated.