Each villager tasked to a Mill gathers 0.67 food per second, which is slower than hunting (0.84 food) until improved by Mill technologies, Home City Cards, and/or native technologies. Up to 10 villagers can work on a Mill at a time, though due to collisions between units it is best to only use 7 per Mill.
Native American civilizations have access to the Farm, which combines the functions of a Mill and a Livestock Pen. the Asian civilizations have the Rice Paddy, which combines the functions of a Mill and an Estate. The African civilizations have access to the Field, which has the same function as the Rice Paddy, but with a slower gathering rate. The Mexicans have the Hacienda, which combines the functions of a Mill, Estate, and Livestock Pen.
Mills placed too closely together (side by side with no space between them) may cause some villagers to "migrate" to the adjacent Mill. This bug also occurs with Estates and, to a lesser degree, Farms and Rice Paddies.
Occasionally, Settlers will get stuck in the Mill or Plantation while collecting resources, making it impossible for them to move and collect. However, this bug appears to be more common in the Definitve Edition.
A mill is used to break grains down into smaller pieces making cooking and baking easier. They were historically powered by wind, water, or livestock. Grain is ground between stones, known as millstones, until it is fine enough to be used for whatever purpose is needed. Flour for bread was made in this manner.
An in-game Mill
German/Russian Mill models (from left to right): Colonial/Fortress Age and Industrial/Imperial Age