Grain mills are used to crush grains in preparation for brewing. Crushing grains is a very important step in the brewing process, as the crush of the grains determines the efficiency of the mashing process. Efficiency is the percentage of fermentables in the grain (by weight) that are absorbed into the wort during the mashing process.
Finely crushed grains result in higher mashing efficiency, but form a poor grain bed that can easily become "stuck", halting the water flowing through the mash tun during the sparging process. This condition is called a "stuck mash". Coarsely crushed grains allow for free flowing of sparge water, but produce lower efficiency.
An ideal grain mill finely crushes the interior of the grains while leaving the grain husks largely intact. These husks then form a grain bed that allows sparging with high efficiency. Dual roller grain mills are considered the best option for home brewers to achieve this type of milling.