Acidity is one of the three most important components of a coffee flavor profile.
A pleasant acidity is something desirable in the cup, because without acidity the coffee tastes bland. In English, a distinction is made here between "acidic" and "sour." Acidic describes a fruity, pleasant acidity, such as strawberries and apples have. Sour, on the other hand, refers to really "sour" foods such as vinegar.
Dark roasted coffee tends to have a lower acidic content because the main source of acidity in coffee is chlorogenic acid, which decomposes when exposed to heat.