She called him back to hand over something he'd forgotten.

Literal

She [topic-は] forgotten-thing [object-を] hand-over [purpose-ために] him [object-を] called-back.

呼び戻す is a compound verb — 呼ぶ (call) + 戻す (return/bring back). 忘れ物 ('something forgotten') is a common compound noun using the productive ~物 suffix.