Tannins

Tannins in wine come from the skins and seeds and produce a sensation of bitterness and the physical sensation of drying in the mouth. The are in fact a subset of Polyphenols and over time they tend to form long chains. Eventually the long chains become too long and the wine precipitates out and forms the sediment you see in old bottles of wine but helping to soften the wine.
The best way to deal with tannins is to eat cheese or meats which will help negate the drying and astringent effects of tannins.