。
One shoe had been left on the doorstep.
Literal
One-side's shoe [subject-が] doorway's stairs [on-に] left-behind was.
残されていた combines the passive of 残す (to leave behind) with ていた (past resulting state) — the shoe 'had been left.' The passive adds a sense of unknown agent: someone left it, but who isn't stated. 戸口 means entrance/doorway.