。
She's coming back at the end of this month.
Literal
She [topic-は] this-month 's end [at-に] returns-and-comes.
終わり is the noun form of 終わる ('to end'), used here as 'the end of' in a temporal sense. The compound verb 帰ってくる ('return-and-come') uses ~てくる to frame the return as motion toward the speaker — she's coming back here. The mirror 帰っていく would mean 'going back away' — the kanji 帰 is the same, but the directional auxiliary flips the perspective entirely. に on 終わり marks the time point of the return.