The mill grinds, both literally and metaphorically. In metaphorical terms, it grinds down the French peasant workforce, stripping them of their humanity with hours of back-breaking, soul-destroying toil. The mill perfectly symbolizes the treatment of the poor in pre-Revolutionary France. The poor, especially the rural poor who form the bulk of the population, have no hope, no realistic means of improving their miserable lives.
Dickens may not present the French Revolution in a particularly flattering light, especially in relation to the wave of bloody violence it unleashed, but he certainly sympathizes with the plight of those who threw in their lot with the Revolution in hope of a better life. By the same token, Dickens, in his heart-rending description of rural poverty, is condemning the indifference of the French aristocracy to the immense suffering that they have inflicted on the peasantry.