Acidity is a natural component of grapes and the acid that is present in the grapes as they are harvested will translate to the acid in the finished wine. While there is a flavor associated with acid, a tartness or sourness, acidity is similar to tannins in that it is more of a feeling than a flavor. While tannins dry your mouth out acidity in wine is what makes your mouth water.
When the grapes are harvested part of what a winemaker is looking for in determining the best time to pick is the balance of acid and sugar. As grapes ripen on the vine the sugar content rises and the acid content decreases. So winemakers are always looking to strike the optimum balance. Because the ripening of the sugars in the grape diminishes the acidity most wines that are high in acid come from cooler climate growing regions. Take Germany for example. Germany is a wine-growing region that exists at the northern extreme of where grapes can grow. Riesling is one of the top grapes grown in Germany and while Riesling is actually grown in a handful of different regions it originated in Germany and Germany still produces some of the best examples of Riesling that you can find. Riesling is a very high acid grape and grown in such a cool climate as that of Germany it can produce light, ethereal, piercingly acidic dry wines. Or, in optimum weather conditions, beautiful balanced sweet wines. Residual sugar and acid play really nicely off one another and are often paired together. Leaving a little bit of residual sugar in the wine is a way to both balance out the natural acidity with out actually making a sweet wine, or make a sweet wine that is still clear and bright and refreshing instead of syrupy sweet.
Acidity is important to wine for a number of different reasons. There is even a process called acidification where winemakers can add in natural grape sugar to make a wine more acidic or to help balance out some of the residual sugar. Acid helps to balance out the flavors, basically a dry wine with out acidity will seem really dull and flat and a sweet wine without any acidity will seem flabby and saccharine. Conversely of course an overly acidic wine will seem overwhelmingly tart and sharp. Acid is also a natural preservative and is part of what helps prevent spoilage when aging wines. And of course acidity is what makes wine such a natural partner for food, and isn’t that really the most important thing. The acidity in a wine will make your mouth water, which will make you want another bite of food, which will make you want another sip of wine. It’s a lovely little cycle.