Acidity of coffee is typically measured by the amount of Chlorogenic Acids (CGAs) in the coffee. Coffee Acidity starts high for an unroasted coffee bean, and typically decreases as the coffee bean is roasted. Good acidity is often described as dry or bright and sparkling. When described this way, the coffee is considered to be high quality to the coffee connoisseur. Bad acidity is often talked about as having bad taste. And, is often thought about as causing stomach sourness.
On the pH scale a value of 7 is considered neutral. Anything above 7 is low in acid, or basic. Anthing lower than 7 is considered acidic. Coffee typically comes in around 5, with higher acidic coffee coming in even lower, and lower acidic coffee coming in towards the neutral value.
Acidity is one of the six typical characteristics that are used to measure the quality of coffee. The other five being body, bitterness, sweetness, aroma, and finish.