Buen provecho is also the equivalent (at least in Mexico) of bon appetit, buon appetito, eet smakelijk: it is something you say before tucking into a meal. It can be shortened to provecho, but this is not very good form.
Interestingly enough, it is also what you are supposed to say when you leave a place where people are still eating, like a restaurant: yes, that's right, you exit the place, and say buen provecho to people you don't know. And they answer ¡Gracias!.
Notice that this etiquette only applies to establishments that are not exceedingly refined, like a torta stand or a comida corrida place.