。
She prepared for her old age.
Literal
She [topic-は] old-age [possessive-の] preparation [object-を] did.
老後 (ろうご, literally 'after aging') means 'old age, retirement years.' 備え (そなえ, 'preparation, provision') is the noun form of 備える. 老後の備え is a set phrase commonly used in financial planning and life-planning contexts.