When you squeeze a slice of lime into your gin & tonic to improve the flavor, you're basically increasing the acid content. Same goes for wine. All wines contain a variety of essential natural acids, mainly tartaric, malic, lactic, and citric. In addition to preventing spoilage during fermentation and aging, acids add tartness to counter the wine's sweetness and give it balance and a crisp, refreshing taste. A wine described as "acid" or "acidic" is typically a wine that is out of balance because the acidity level is too high. Wines that lack enough acid can taste flabby or flat. FYI, the taste buds for sensing acids (which come through as sour) are on the sides of your mouth and tongue.