Must is unfiltered fruit juice that will contain the seeds, skins and stems.
The juice is usually grape.
Juice will naturally ferment and turn to alcohol: when keeping it at the “must” stage is desired, it must be either frozen, or pasteurized to prevent that.
Must is used to make wine from. It is also still used as a cooking ingredient in some Greek dishes, and in Italian Mostaccioli Cookies. It can act as both a liquid and a sweetener in a recipe.
You can buy must in which fermentation has just begun, but then been pasteurized to stop it from fermenting any further. It is sold in bottles.
When the solid particles — the seeds, skins and stems — are strained out, they are referred to collectively as the “pomace.”
Typically, must is a freshly-pressed grape juice that has been set out for up to 24 hours, and in which fermentation has just barely begun. (While it isn’t deliberately let set around for 24 hours, it often is.)
It is less than 1% alcohol. It can be red or white, depending of course of which grapes it was pressed from.
Literature & Lore
The Roman Catholic Church has debated what “must” (they call it “mustum” in their official communications) is. In the Catholic church, the priest drinks — and has to drink — the wine during the Communion services. The concern is in providing alcohol to priests who have had a problem with alcohol, and the question is, “can must be used instead?”
The Church won’t accept pasteurized must. They say that pasteurization evaporates what little alcohol there was in the must, and without that alcohol, the liquid is completely something else than what is intended to be consecrated for the ceremony. The Church will, though, allow must that is frozen. Freezing arrests the fermentation without removing the traces of alcohol.