。
She slid down the ladder.
Literal
She [topic-は] ladder [object-を] slid-down.
滑り落ちる is a compound of 滑る ('slip, slide') and 落ちる ('fall'), giving a single verb meaning 'slide and fall, slip down.' The masu-stem of the first verb fuses with a second verb of motion or completion to express manner + result — the same machinery behind 駆け上がる ('run up'), 飛び降りる ('jump down'), 持ち帰る ('take back'). The を here is the path-marking を: 落ちる takes から for source, but compounds with motion-verb objects often allow を for the route or surface traversed.