What does acidity mean in terms of coffee? How does it affect taste?
Acidity in coffee results in a brightness of flavor.  Higher Grounds describes it as "dry, bright &amp; sparkling sensation". Sweet Maria, a purveyor of unroasted beans, describes it as "bright, clear, snappy, dry, clean, winey, etc."

In truth, you would need to sample a variety of coffees with different characteristics and develop your own sense memory, because it is extremely difficult to describe flavors.

A nice Ethiopian full city roast will have good acidity when compared to say a French roasted anything.  In general, coffee grown at higher altitude will have more acidity than lower grown coffee; and lighter roasted coffee will retain more of its acidity than darker roasted coffee.

See also:


Kaladi Coffee article on coffee acidity
Higher Grown: Coffee Acidity: the Science &amp; the Experience