。
She'll be back soon.
Literal
[topic-は] before-long return-come [probably-でしょう].
戻ってくる combines 戻る ('return') with the deictic auxiliary ~てくる ('come [doing something]'), giving 'come back toward [the speaker/reference point].' The auxiliary marks the direction of return as toward where the speaker is. でしょう is the polite speculative ('probably will'). Without ~てくる the same idea is expressed by plain 戻る ('return'), which leaves the directional perspective open — 戻ってくる is the speaker's-side version, 戻っていく ('go back away') the away-side. Japanese is unusually expressive about deictic motion.