This phrase does not have a direct English translation. Sometimes you’ll see “Bon appétit!” as a translation, however:

“Bon appétit!” is usually said by a person serving food to another, or if you’re suggesting to someone to enjoy their food.

”잘 먹겠습니다!” is slightly different in the sense that two friends can say it together in unison, after their food has arrived.

It’s not a command or a suggestion or a wish upon someone - it’s moreso a declaraction of thanks that the party is about to enjoy a meal.