。
She kept the memory of her dead husband locked in her heart.
Literal
She [topic-は] dead husband of memory [object-を] in-chest was-keeping-secret.
胸に秘める ('to keep [a feeling] secretly in one's heart,' literally 'hide in the chest') is a literary, almost poetic expression for harboring private emotions or memories. The 胸 ('chest') is the Japanese seat of emotion and inner feeling — many idioms locate feelings there: 胸が痛む ('the heart aches'), 胸がいっぱい ('the heart is full'). 思い出 ('memory, recollection') is a noun built on the verb 思い出す ('to recall'), referring specifically to remembered moments — distinct from 記憶 ('memory as a faculty'). The whole sentence has a wistful, almost mournful tone.