A more thorough explanation:

A mill is a machine that grinds corn, grain, or other substances using a wheel and circular motion. The substance that is ground in a mill is sometimes called grist, especially when it is a grain. A mill can also refer to the building in which the grinding is performed, along with the site, dam, or other items connected with the mill.

For example, a farmer might take their grain to a local mill to have it ground into flour. The mill would use a wheel to grind the grain into a fine powder, which could then be used to make bread or other baked goods.

The term "mill" can also refer to the tenth part of a cent. For example, if something costs $1.50, it would be 150 mills.

Courts sometimes use the grinding process of a mill as a metaphor for the judicial process. For instance, suits to collect on promissory notes are grist for the summary-judgment mill because the material facts in such cases are often undisputed. This means that the court can quickly and easily make a decision based on the evidence presented.