When a wine is aged, it has been purposefully kept somewhere for many years. This is done in a bottle or a barrel. 

Wines that have high alcohol, acidity, tannins, or sugar content can be aged for longer. Some wines, especially delicate wines, cannot be aged for long while others can be aged for decades. 

Aging helps some wines develop their aromas. Aging also mellows tannins and alcohol, making the wine taste more delicate and nuanced.