Every anime meal scene: hands together, eyes closed — いただきます！ The subtitles say “let's eat!” or “thanks for the food”. Both are half right.
What it literally means
いただきます is the humble form of “I receive”. Before eating, you're thanking everyone and everything the meal came from — the cook, the farmer, and the ingredients themselves. It's gratitude compressed into one word, which is why there's no clean English translation.
They come as a pair: いただきます opens the meal, ごちそうさまでした closes it. Among friends you can drop the でした — ごちそうさま！
もぐもぐ…
*already eating*
いただきますは？
Aren't you forgetting something?
…いただきます。
When to use it
Any meal — many people say it even when eating alone. If someone treats you, ごちそうさまでした doubles as your thank-you to them. And because it literally means “I humbly receive”, you'll also hear it when accepting a gift or a snack.