The word molían is the imperfect form of moler in the third person plural.
See the full moler conjugation

moler

Aquí es donde molían la harina.
This is where they ground the flour.
Los centenares de gente molían alrededor del centro de la blanco en ese lado del edificio.
Hundreds of people were milling around the Target Center on that side of the building.
Preparaban sopa, masa y pan a base de nueces de los árboles de la región que molían con una especie de molcajate o metate.
Acorns from several species of oaks were ground with a mortar and pestle to provide soup, mush and bread.
El agua de un canal de madera moviliza su rueda de paletas y en su interior se pueden ver las maquinarias y las grandes piedras que molían la harina.
The water of a wooden canal moves its wheels and its inside shelters the machinery and the large rocks that used to grind the flour.
La común, la que molían con frecuencia, era la versión de Carlos Dante y Julio Martel, con la limitada y popular orquesta de Alfredo De Angelis, y no sé si había otra interpretación de bar.
The customary recording, the one which was played frequently, was the rendition by Carlos Dante and Julio Martel, with the anodyne and popular orchestra led by Alfredo De Angelis, and I don't know if there was another rendering for playing at barrooms.
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