。
Once mother comes back, I'll head out.
Literal
Mother [subject-が] came-back [when-たら] I [topic-は] leave (polite).
~たら is the temporal/conditional 'when/if' — here clearly 'when X happens, Y will happen.' 戻ってくる is a compound verb (戻る + 来る) adding the 'toward here' direction to 'return': 'come back' rather than just 'return.' The ~て~くる auxiliary is a productive pattern for adding directionality toward the speaker.