An aperitif is an alcoholic drink served that is typically served before a meal. There are numerous types, be it a dry sparkling wine, Fino sherry, vermouth or even pastis. An aperitif can also be a cocktail such as a kir royale (cassis and white wine) or a Negroni.
Aperitif is sometimes used to refer to the light foods that are served as a snack before the meal; this aperitif refers to the custom of a drink a light bite to eat before the main courses.