A wine's vintage is the year in which the grapes were harvested. This is clearly indicated on most wine labels.
Depending on the region, a wine's vintage may affect the wine's final quality more or less. Regions with greater year-to-year weather variation are going to have greater variations in quality each year. 

Vintage can also be used to determine the optimal time to drink a wine -- big, bold wines with high acidity and high tannins, such as Barolo, are better consumed 5+ years after the vintage date, whereas young, fresh, fruity wines, such as Sauvignon Blanc, should be consumed within 2 years of the vintage date. 

If a wine is listed as "non-vintage" (typically used in the case of sparkling wines), it is made by blending multiple years together to achieve a consistent, house style. These wines are typically a good value. For example, a common non-vintage wine is Champagne labeled simply as “N.V.”