。
She stepped outside for a breath of fresh air.
Literal
She [topic-は] fresh air [object-を] one-breath inhaling [to-に] outside [to-へ] went-out.
一息 (ひといき, 'one breath') combined with 吸う ('to inhale') creates the purpose: 'to take one breath of fresh air.' The purpose-of-motion に connects this to 表へ出た ('went outside'). 表 (おもて) means 'outside, outdoors' — literally 'the front/surface,' used for stepping out of a building into the open air.