From the L.A. Times
When most cooks read "season to taste," they automatically reach for the salt shaker. That's not a bad start: A judicious sprinkling with salt will awaken many a dull dish. But if you stop there, many times you'll be missing a key ingredient. Because just as a little salt unlocks flavor, so can a few drops of acidity.
Add a shot of vinegar to a simple stew of white beans and shrimp and notice how the seemingly simple, earthy flavor of the beans suddenly gains definition and complexity. Do the same thing with a soup of puréed winter squash and see how a dish that once was dominated by rich and sweet now has a round, full fruit character.
Though the results may be similar, salt and acidity work slightly differently. Salt is a flavor potentiator -- in other words, it works chemically to make other flavors taste more of themselves. Acidity works as seasoning by giving a dish backbone or structure, which allows other flavors to stand out and shine.