Think "pucker factor" for the mouth. Tannin is the bitter, zesty element in wine that softens with aging yet is crucial for the maturing of red wines. It comes from a grape's seeds, stem, and skin, as well as from the wood barrels used for maturation (the newer the barrel, the greater the degree of tannin imparted). If the wine makes your mouth pucker and your eyes water, it's usually due to high levels of tannin. FYI, most of today's commercial red wines have a precise tannic balance and need little, if any, aging.