Miller is the skill associated with the milling labor. Millers mill plants at either a quern or millstone, placing the resulting goods into bags - which produces one unit of dye, flour or sugar per item milled, as well as 1 or 2 seeds. Milled items have no extra quality associated with them. Millers can also mill seeds/nuts into seed paste, a precursor to making oil - only the seeds of plants with an associated oil can be milled.
A quern is powered by the dwarf themself, while a millstone uses outside mechanical power (either a water wheel or a windmill). Using a millstone is, obviously, quicker, though it is also more difficult to set up.