Tannins are a group of bitter and astringent compounds that are found throughout nature. Not only are they found in wines, but also in your tea, cranberry juice, rhubarbs, and plums.
What are tannins responsible for? Basically, they are like a defense mechanism for plants. They make an astringent and bitter taste in the mouth of any animal who dares eat them. Humans however don’t seem to mind this flavour too much! Some products specifically are known just for their tannins, tea, some chocolates, and of course wine.
So in terms of wine, what do tannins do? The compounds that makeup tannins actively bind to the proteins in your saliva to create a dry mouth feel. This can help when pairing wines and food as a particularly high tannin red wine would make eating a steak more enjoyable as you would taste the juices and flavours in the steak more with a dryer tongue.. sounds quite odd but try it!