Acidity in coffee results in a brightness of flavor. Higher Grounds describes it as "dry, bright & 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.