。
She confided her terrible secret.
Literal
She [topic-は] her own terrible secret [object-を] opened-up.
打ち明ける is a compound verb (打つ 'strike' + 開ける 'open') with the figurative meaning 'to confide, to open up about, to disclose (something private).' The image is of breaking open something sealed — a fitting metaphor for revealing a closely-held secret. 恐ろしい ('terrible, frightening, dreadful') is a strong i-adjective for fear-inducing things; combined with 秘密, it marks the secret as something whose disclosure carries weight — perhaps a confession of wrongdoing or a guarded family truth.