provecho is short for buen provecho, and it is an expression used in Mexico to wish people a good meal.
As such, I can't really find an English language translation for it. It is a bit like "bon appetit" in French, "buon appetito" in Italian, "eet smakelijk" in Dutch ...
The Mexican provecho has another use which I find quite interesting: when you are leaving a restaurant, especially a small cheap tortas or taco place, you say "¡provecho !" to the people that are sitting at their table (and still eating).
They are expected to answer "¡Gracias !".
It all forms part of Mexican manners