。
She let me know about the departure.
Literal
She [topic-は] me [to-に] departure's matter [object-を] informed.
知らせる is the causative-derived verb 'to let someone know, to inform' — literally 'cause [someone] to know.' The relation 知る → 知らせる ('know' → 'cause to know') parallels 見る → 見せる ('see' → 'cause to see' = 'show'), 食べる → 食べさせる ('eat' → 'cause to eat' = 'feed'), and 飲む → 飲ませる ('drink' → 'cause to drink' = 'give to drink'). Several of these have lexicalized so thoroughly that they no longer feel like causatives — 知らせる is a stand-alone verb in everyday speech.