。
She was about to go out.
Literal
She [topic-は] go-out [about to-ようとしていた].
~ようとしていた is the past form of ~ようとしている (about to do), placing the imminent action in the past — she was on the verge of going out at that moment. This past-tense version is common in narratives to set up what happened next: 'she was just about to leave when...' 出かける is a versatile verb meaning to go out or head out, used for everything from casual errands to formal outings.