。
She went down the stairs.
Literal
She [topic-は] stairs [path-を] descend-went.
降りていく fuses 降りる with the directional auxiliary ~ていく, planting the speaker at the top of the stairs while she descends away. This is the mirror image of 降りてくる: same descent, but the camera stays put and the subject recedes into the distance. The を on 階段 marks the path traversed (just as it does with 歩く, 渡る, 走る), not a direct object — a small but useful piece of Japanese particle behavior. Past form ~ていった makes the whole motion completed at some past moment.