Acidity is a natural ingredient of wine. The content of acidity changes during the ripening process: the riper the grapes are, the more they contain natural sugars and the less they contain acidity.
This is the reason why we put emphasis on very ripe grapes. They go onto the press already having a naturally low level of acidity.
Additionally a wine’s acidity depends on the grape variety. Generally the acidity of Riesling is higher than that of other white grape varieties.
The higher the acidity of a wine is, the higher is the risk that it will cause problems for sensitive people. Persons having a tender stomach use to react on high acidity with heartburn.
If you are sensitive to a wine's acidity, try to choose one having less acidity than 6 g/l. It also helps to eat something while enjoying wine.
Referring to wine the terms “dry” and “sour” are often mixed up. “Dry” simply means “not sweet”; it has nothing to do with a wine’s acidity.