Short definition
Freshly pressed grape juice that includes skins, seeds, and stems before or during fermentation.
Before fermentation begins, you have must — essentially grape juice with all its solids. Red must includes skins for color and tannin extraction. White must is typically pressed off the skins immediately. The sugar content of must determines the potential alcohol of the finished wine. Winemakers test must with a refractometer to measure sugar in Brix degrees.