Definition - What does Smooth mean?
Smooth is a term used to describe a wine that has a smooth tactile sensation. More specifically, the term is used to describe the characteristic of a wine which has undergone a secondary fermentation (malolactic fermentation/MLF) This is the process by which the sharp malic acid, naturally existing in wine is "fermented" into lactic acid, a softer, "smoother" acid.
WineFrog explains Smooth
From a technical standpoint, "smooth" is an adjective used to describe the texture of the acidity in a wine. Smooth is used to point out that the wine has undergone malolactic fermentation (MLF). It is technically not a fermentation, but actually an inoculation of a wine with a certain bacteria, Oenococcus oeni. This bacteria is able to convert naturally existing malic acid in wine which is "harsh" or "tart", like that of a green apple, into a "smoother" acid, lactic acid, like that from buttermilk and creamy cheeses. This transforms the palate of a wine, making it not only smoother in its acidity profile, but it also adds body and creates a supple sensation on the palate.