Acidity is a natural ingredient of wine. During the ripening process of the grapes their content of acidity changes: increasing ripeness means increasing content of natural sugars and decreasing acidity.
Briefly speaking: The higher the ripeness, the lower the 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 with the wine.
In combination with wine the words “dry” and “sour” are often mixed up. The true meaning of dry is “not sweet” and has nothing to do with a wine’s acidity.