غُثَاءٌ denotes froth or insubstantial debris borne on a flood. كَغُثَاءِ السَّيْلِ uses كَـ for the simile “like the froth of a flood,” contrasting numbers with effective strength.
بَلْ corrects the prior assumption: “rather.” وَلَكِنَّكُمْ introduces the contrasting point: “but you…”
The imagery describes communal vulnerability; it does not remove the dignity or rights of any individual experiencing weakness.