。
She was getting ready to leave the house.
Literal
She [topic-は] house [object-を] leave preparation [object-を] was-doing.
Two pieces work together. 家を出る ('leave the house') uses を to mark the place being departed from, the same way it does in 学校を卒業する ('graduate from school') or 電車を降りる ('get off the train') — a special departure-marking use of を. The verbal noun 準備 ('preparation') paired with をする (or just する) is the standard way to express 'prepare for / make preparations for.' The polite past progressive ~ていました places the preparation in the middle of unfolding at some past moment.