Tannin is acidic, which derives from grape skin and grape seed. The fermentation process of Red wine has to reserve grape skin. In the process, wine liquid would abstract some tannin from oak barrel. Based on chemical structure, tannin from grape skin is called condensed tannin and that from oak barrel is called hydrolysable tannin.
Tannin is soul of red wine, whose framework is set up to make wine structure stable and full bodied. It effectively polymerizes stable pigment substance to present perfect and viable color and sets off chemical reactions with other substance in the wine liquid to get new ones and to improve the complexity of wine.
If a wine is lack of tannin, the body would be lighter and taste softer. Of course, we can’t easily tell the higher tannin the better and we could suggest well balanced wines as below: