A small tub of tannin.
It adds astringency- Whats that then?
When you chew a grape skin or pip you get a sort of dry feeling in your mouth, that's the tannin that causes that and it's called astringency. A cup of tea has the same effect but milder, we like it in a small amount in our tea and wine.
So, we add it to country wines to give them a bit of astringency-Got that? It makes it nicer, but don't add too much!