。
She visited her husband in prison.
Literal
She [topic-は] prison [in-に] is-located husband [object-を] visited.
A clean example of a relative clause modifying a noun: 刑務所にいる ('is in prison') sits in front of 夫 ('husband') and modifies it, giving 'the husband who is in prison.' Japanese builds these clauses in front of the noun with no relative pronoun (no equivalent of 'who/which/that') — just the verb in plain form leaning directly on the noun. に here marks the location where someone exists, paired with いる for animate beings (vs. ある for inanimate ones).