Tannin is the substance that provides bite, firmness and character to a wine. 
It needs to be present at the right level to achieve the correct balance. 
Wine tannin, a reddish-brown powder, is extracted from grape skins and should be added to musts prepared from ingredients that are naturally low in tannins. 
Whilst red fruits such as blackberries, elderberries and grapes should all contain adequate levels, wines made from vegetables, grains, flowers and white fruit, with the exception of apples, usually benefit from the addition of tannin. 
The correct level helps to clear the wine by preciptating out any proteins present and will also improve the wine's bouquet & keeping qualities.