What acidity means
Acidity is the sourness or sharpness in food and drink, caused by the presence of acids. Lemons, limes, vinegar, tomatoes, many fruits, wine, yoghurt and fermented foods all have noticeable acidity. In cooking it matters mainly for flavour balance. A dish with too little acidity can taste flat, heavy or overly sweet, while too much makes it harsh and puckering.
Cooks adjust acidity just as they adjust salt, adding a squeeze of citrus, a splash of vinegar or some wine to brighten a dish, or softening sharpness with a little sugar, butter, cream or longer cooking. Acidity also affects how food behaves. It can slow the softening of vegetables and beans, help set pectin in jams, make marinades work and react with raising agents in baking. In drinks, especially wine and coffee, acidity is described as freshness or brightness and is considered a sign of good quality rather than a fault.
Many fruits and vegetables vary in acidity from batch to batch, which is why recipes so often say to taste and adjust at the end. You will see the word in recipes, wine and coffee tasting notes, and cooking guides that explain how to balance flavour.