Tannins originate from the skins of the fruit, the stems and, to a much lesser extent, oak of the barrels. They impart a ‘puckery’, astringent or sometimes bitter sensation to the wine. They also contribute to the structure and body of the wine. (redwinebuzz.com)

A better wine, then, isn’t bitter. Essentially, the tannins provide the structure for the wine to age, but a good, aged wine’s flavor isn’t overwhelmed by tannins. (AgroTropical Spain)

Pretty much all wine, irrespective of how its made, where its made or what grapes its made with, has certain things in common – it has fruit flavourings, it has acid, it has alcohol and, if is red, it has tannin. That’s it, that’s structure: Fruit, acid alcohol and Tannin. When people talk about structure they are talking about the interplay of these three things. (Wine guide)