Definition - What does Grape Skin mean?
In the context of wine grape anatomy, the grape skin is the outer layer of the fruit of the wine grape. In addition to protecting the pulp and juice, the skin plays an important role in both the color and overall flavor of the wine that it produces. The grape skin gives wine flavor, aroma, tannin, determines its color and adds natural yeast to the wine based on the varietal.
WineFrog explains Grape Skin
Grape skins are not only the protective amour for the juice and pulp, they are an integral component in the fermentation process; they determine the color of the wine and contribute many of the flavor and aroma characteristics of the wine. Grapes that have a milky or frosty appearance, also known as bloom, contribute enzymes and airborne yeast to the wine.
Wines get their final color from the skins of the varietals used to make it, as the pulp and juice are clear or pale-yellow/green in color when not introduced to the skins during pressing or crushing. After harvest, the grapes are crushed to become a must, and this marks the beginning of fermentation. How long the must, which contains the skins, ferments is up to the winemaker and depends on the type of wine being made. The longer the skins have contact with the juice, the more tannins, flavors and color (from red wine grapes) will be in the wine.