Allowing a wine to “breathe” means exposing it to air before serving. This exposure to oxygen can help release the aromas and enrich the flavors of the wine.
“Breathing” begins the minute you remove the cork or cap. Allowing a wine to rest in the bottle with the cork removed requires much more time to be effective because there is such a small amount of the wine surface exposed to oxygen. Pouring the wine and swirling it around in a glass provides more exposure and can improve as you drink it. For a much greater effect, the wine can be decanted, which means pouring it from the bottle into another glass vessel where it rests or is swirled before drinking or returning it to the bottle. Using a decanter is also used to help remove sediments from the wine.
Young bolder red wines, especially those that are high in tannins, such as Cabernet Sauvignon, Zinfandel, Bordeaux and other Rhône Valley wines, taste better because aeration softens the tannins and enhance the taste. These can take 1-2 hours to properly “breathe”.
Mature red wines are typically more mellow and may benefit from 30 minutes of breathing if even needed. Very old wines do not require aeration.
You should be careful when choosing to aerate white wines since not all of them need it. Delicate white wine, rose, champagne, and sparkling wines are not aerated and are opened just before serving.
Be careful though. Extended exposure to air has a negative effect on the wine. It can deteriorate older delicate wines, and cause wines to taste or smell vinegary. This can be avoided by only aerating or decanting the amount of wine that you plan to serve.
If you like to pour and drink immediately, and have no patience for the breathing process, you can use a commercial aerator to serve the wine. They actually do work.