。
She tried to hide her mistake from us.
Literal
She [topic-は] her own mistake [object-を] us [from-に] hide [tried to-ようとした].
に here marks the recipient of the hiding — the person from whom something is concealed: 我々に隠す ('hide from us'). 我々 is the formal/literary first-person plural pronoun, more elevated than 私たち, common in speeches and written narrative. ~ようとする ('to try to') marks an attempt; the past ~ようとした reads as 'tried to but...' (often implying it didn't fully succeed). The 我々 in particular gives the line a slightly old-fashioned, official-sounding air.