A: Tannins are polyphenols, naturally present in all plants and fruits and therefore also in grapes (Vitis Vinifera). In particular, they are contained in the grape seeds and in grape skins and it can also be found in wooden barrels.
Q: How does it taste like ?
A: It doesn't really have any flavour. But have you ever brew your tea for too long? That sensation that dries your mouth is similar to the "tannin".
It is also a strong antioxidant: that's why red wines that contain higher tannin tend to age better.
In young wines, tannins have shorter molecules, which confer an astringent and “aggressive” flavour. With time, they tend to polymerise, for a more “round” and velvety taste.
