Have you ever heard someone describe a red wine as being chalky? This is not a reference to an aroma or flavour but a way to describe how the wine feels in the mouth; as the industry calls it "moutfeel". Chalky is just one descriptor used for a group of tactile sensations is referred to as astringency. Its the same sensation you experience when drinking green or black tea that has been brewed too long.
Astringency describes the drying, puckering, chalky, rough and grainy feeling in the mouth caused by tannins in wine. Tannins come from grape skin, seeds, and stalks as well as oak treatment and the amount of tannin in the final product is dependent on too many factors to explain here.
So, what creates this drying sensation? It's actually caused by the interaction between tannins and proteins in saliva and other molecules in the mouth. This binding process causes moitsture to be drawn from the tissues and results in astringency.
The opposite of astringency is when the mouthfeel of wine is smooth, silky and velvety. This can be due to either a lack of tannin in wine or changes to the molecular structure through various chemical processes, including aging.
This is why we put a young wine that tastes slightly rough into the cellar. Over time, tannins “soften” as they polymerize, or form long chains with each other. That's why we say that red wines mellow with age.