Tannins

Tannins - astringents of tannic acid, which are contained in red wine, due to their origin of the skin of the fruit and partly to bones and scallops (twigs). 

If the winemaker leaves the juice in contact with the peel for a long time, then the wine becomes more tannic (it leaves a tart taste in the mouth like strong tea).

In the case of a short contact of the juice with the skin, the wine turns out to be softer, more delicate.