Literal translation
millstones are falling
Meaning
it's raining extremely hard
Explanation
Mediterranean rains, sporadic and brief, hit with an intensity that several Provençal expressions try to render by comparing the rain to something massive. Here it's the millstone, an enormous multi-ton wheel, once one of the most imposing everyday human constructions, oil and wheat mills being plentiful in Provence. People also say it's raining buckets (Provençal plòu à farrat), more common but less vivid, or, in the Rhône valley, plòu à cop d'ase, "it's raining donkey-blows".