Something that's acidic has a sour or sharp taste. Lemonade, especially if it's light on the sugar, tastes very acidic.
Citrus fruits have a typically acidic flavor, and they qualify as acidic in the scientific definition of the word as well. In chemistry, something that's acidic has a low pH, as opposed to a base, which has a higher pH. Various kinds of acids and vinegar are also acidic. The word comes from acid, which originally meant "of the taste of vinegar," from the Latin word acidus, "sour or sharp."