Fining a wine is the process of clarifying and stabilizing a wine by adding a specific material to the wine that will attract and absorb unwanted particles, removing them from the wine.
Sound confusing? A bit. After wine finishes the fermentation and ageing process in the winery/cellar and before it is bottles, many wine makers will use a material or fining agent in the wine to absorb molecules that may make the wine unstable. The materials used can range from egg whites to fish bladder to bentonite (a specific clay material), but very few traces of the materials remain as their job is to simply pass through the wine, removing the unwanted molecules. Fining materials are not left in the wine itself.
So why is fining used? For young, inexpensive, ready-to-drink wines, fining helps clarify the wine quickly, so it can be bottled and not turn hazy or cloudy, as well as not have any unwanted tartrates or bitter tannins. Some of these particles will naturally fall out of the wine, but only with time, so wines bottled young are typically fined. Fine wines, those that need more ageing and have more complexity and character, are often not fined. Fining removes those unwanted particles, but it often removes the wanted particles that add complexity as well. Many of the unwanted or unstable particles naturally go away just by allowing the wine to age and sit before bottling.
Fining agents include: egg whites, isinglass (proteins acquired from the bladders of sturgeon), casein (a protein found in milk), and bentonite, a form of clay.
If you run across an “unfined” wine, it does not imply that the wine is brimming with bacteria. In fact, it may mean you have a very complex and interesting wine on your hands, and one worth trying.