。
She confided a secret to us.
Literal
She [topic-は] us [to-に] secret [object-を] confided.
打ち明ける is a compound verb formed from 打つ ('strike') + 開ける ('open'), literally 'strike open' — a vivid metaphor for opening one's heart and revealing what's inside. The に here marks the recipient of the disclosure, the standard Japanese pattern for verbs of giving and telling: ~に教える ('teach to'), ~に話す ('speak to'), ~に言う ('say to'). Compare with the lighter 言う ('say') or 話す ('tell') — 打ち明ける specifically connotes revealing something personal or hidden.