。
She was waiting in front of the building.
Literal
She [topic-は] that building's front [at-で] was-waiting.
の前で stacks two pieces a learner meets early: の as a genitive linker giving '[building's] front' and で marking the place where the action happens. The verb form 待っていた is the past durative of ~ている: not just 'waited' (待った) but 'was in the act of waiting'. That distinction matters in Japanese, where simple past ~た often implies a one-shot completed event, while ~ていた zooms in on a stretched-out ongoing state — the speaker's interest is in the duration, not the eventual end of the wait.