'Eat well / Have a good meal.' => 맛있게 먹어 / 많이 먹어.
- 맛있게: adverb of 맛있다(delicious). means "enjoying it", "with gusto/relish", etc.
- 먹어: imperative of 먹다(eat), so it is just "eat!".
- 많이: literally "in good quantity; a lot", but really means "heartily, as much as you like".