Tannins are astringent, bitter compounds found in plants.  Tannins cause your mouth to feel dry and pucker up.  The manipulation of tannin concentration plays an important role in aging of wine and ripening fruit.

Tannins are also important to the flavor development and health benefits in chocolate...

Chocolate liquor contains about 6% tannins, significantly contributing to the bitterness of chocolate.  “Bitter” as a flavor descriptor is not always a bad thing.  It is true that sometimes bitter can refer to negative characteristic coming from improper fermenting or roasting.  But normal amounts of tannins intensify chocolate flavor, especially at the finish. 
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