Grist are the grains that go into the mash. These are all the grains in the recipe: malted, flaked, roasted, etc. You crush these grains to prepare them for the mash. The crush is important; it affects the amount of sugars extracted (efficiency), the ease with which the liquid and solids are separated (lautering), and even the flavour due to tannins being leached from the grain husks.
The goal of the perfect crush (note “crushed” and not “ground”—grinding would be bad, read on) is to pulverize the inner part of the grain where the starches are while leaving the outer husk intact. It sounds almost impossible, but you can do it. The use of a multi-roller grain mill designed for brewing applications is ideal. These mills come in different configurations from the 2-roller type for home use to the multi-storey 6-roller type for commercial use. (Let’s just ignore the large-scale mills.)
The 2-roller type of mill is pretty typical for home brewers. Its basic construction is two rollers that are set parallel to each other below a grain hopper. The mill is operated by a crank (hand operated or motorized, often by an electric drill), which turns the rollers. The grain is feed into the rollers, which are set close enough to crush the grain without destroying the husks (usually there is a “gap” adjustment knob). The grain is crushed and falls out the bottom into a waiting bucket.
If the crush isn’t fine enough all the starches won’t be exposed and the amount of sugar removed will be too low. If the crush is too fine then the husks will be torn up causing problems with removing the liquid from the solids as the husks act as a filter and also the torn husks can leach tannins into the wort, causing undesirable flavours.
The other type of mill that is sometimes used is called a Corona Mill or a Victorian Mill. This mill uses two plates that rub against each other to crush/grind. This type of mill is designed to make flour rather then for brewing so if you’re not careful you could tear your husks, or worse. Though you can be successful with this type of mill, I don’t recommend it.