To keep a promise is to do exactly what you said you would do. Not more, not a substitute — the actual thing, on the actual day.
Keep is the holding verb, so the picture is of carrying something safely for a while rather than performing a duty, which is why keep your word works the same way and honour it sounds like a contract. It takes to: she kept a promise to her brother. Break is its opposite and the pairing is fixed — one you did not keep is broken, never lost.
1phrasekeep a promiseTo do exactly what you said you would do.
“Children remember whether you keep a promise.”
synonyms: keep your word
keep a promise is an act of doing what you said.
Practised in Common Word Pairs