Said before eating — roughly "I humbly receive."
Not "bon appétit." It is a humble verb form directed at everyone and everything that brought the food to the table — the cook, the farmer, the animal. Said even when eating alone. The counterpart after the meal is ごちそうさまでした.
Skipping it at someone's home reads as genuinely rude, in a way skipping "bon appétit" never does in French.
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