Acidity often has a bad reputation because folks associate acidity with sour or bitter tastes. However, when we are determining tasting notes for coffee, we are evaluating the tang or the sharpness of the coffee as it hits the flavor receptors on the tongue and mouth.
When it is offset with a combination of other tastes, acidity can make or break the final flavor. Think of your favorite salad dressing, the acidity is used to balance the bitterness of the greens, the sweetness and saltiness of the tomatoes and other mix-ins, and it cuts through the heaviness of the oil or fat. A recipe that is too sweet or too salty can benefit from adding a little acidity to it to balance it out.
In coffee acidity is usually an expression of the fruit flavors that are tasted. Common acidic attributes are citrus, zippy, crisp, juicy, vibrant, bright, tart, balanced, gentle, and smooth. Think of the tartness of a green apple or the tang of a ripe tangerine.
As you taste your coffee, think of how the flavors hit the different points in your mouth and tongue:
Is there a sharpness on the tip of your tongue or where your jaw meets your ear?
Is the sharpness pronounced and present, or soft and delicate?
Is it complementary or is it too overpowering for your preference?