‘Letting wine breathe’ means to expose it to fresh air. This causes the wine to mix with the air, and will cause the wine to begin oxidizing. Depending on the wine’s type and age, oxidation can cause different effects, both desired and undesired.
For a younger wine, breathing is normally not needed, as most of the wines flavors and characteristics will be immediately present upon opening the bottle.
For an older wine, which has had time to age in the bottle, and which has been exposed to slight amounts of oxygen that seeped in through the cork, a longer amount of breathing may be necessary. Sometimes, allowing an older wine to breath for 10 or 15 minutes will allow its deeper flavors, aromas, and characteristics to show through, which otherwise might not have developed had the wine been opened and then immediately poured.
There are several ways to allow a wine to breathe:
1. uncorking the bottle and letting it sit out for a few minutes
2. pouring it into a decanter (a specially-designed glass or crystal container that permits a maximum surface area of the wine to be exposed to air)
3. pouring the wine into a glass, and waiting a few minutes before drinking it.