Small izakaya, second visit this week. You order two things. Three things arrive.
You start to explain, in careful Japanese, that there has been a mistake.
The owner waves a hand without turning round: "サービス。"
You sit back down. You are now, whether you realise it or not, a regular.
In English, service is what you receive in a restaurant and what you tip for. In Japanese, サービス has narrowed to one glorious meaning: something given free, as a gesture.
You will meet it as サービスです (it's on the house), サービス品 (a free/discounted item), and the beautiful compound サービス残業 (saabisu zangyou) — "service overtime", meaning unpaid overtime, which is the same word doing something far less charming.
It matters because Japan does not tip. There is no mechanism for you to give a little extra to say thank you, and trying will cause genuine distress. So the small generosities travel the other way — from the shop to you — and サービス is the word that marks them.
In a tipping culture, extra kindness flows from the customer to the staff, and it is measured in money. Japan deleted that channel entirely — and the kindness reversed direction. The free plate is not marketing. It is the only remaining way for the person behind the counter to say they are pleased you came, and they have exactly one word for it.