Bon Appétit is the French for 'Enjoy your meal'. It was also used in many other places such as the name of a magazine. But if you heard this or someone wrote it in an email or a letter, just know that it means enjoy your meal.
P.S. the following example is a real life quote that made me find and
add this word in the dicionary:
Student:thanks for the meal today,
believe it or not, today I
didnt have anything for breakfast and not much for dinner the night
before, so that muffins were quite satisfying. :)
And
I never thought of a strawberry that deep in my entire life.
Teacher: No doubt
there'll probably be other unexpected things this quarter, but
I don't know how many of them will be food oriented. "Bon Appetit" regardless.
