This coffee is basically an espresso or ristretto coffee with a splash of alcohol added to it, usually Grappa or Sambuca.
The word corretto means, “to correct”. In this instance the espresso coffee needs to be slightly corrected with something and that something is alcohol.
This all came about because many years ago almost all “bars” (Italian cafes) in Italy were not permitted to serve alcohol without a license and still aren’t. To get around this the customer would come into an Italian bar and ask for a caffè corretto and the barperson would lean down behind the counter and grab a bottle of alcohol (usually Grappa) and add some to the espresso coffee without anyone noticing.
This coffee is popular on a cold winter night in cities like Milan and is great after a pizza or heavy meal to aid digestion. The alcohol warms you and the coffee helps you to digest – together this is a perfect coffee to have after a meal in winter.