。
She waved at us.
Literal
She [topic-は] us [to-に] hand [object-を] waved.
手を振る is the standard collocation for waving — literally 'to shake the hand.' に here marks the direction of the wave (the people the gesture is aimed at), parallel to に in other directional uses: ~に手紙を送る ('send a letter to'), ~に向かって ('facing toward'). 振る is a versatile verb covering a wide range of shaking and swinging motions — also used in 首を振る ('shake one's head' = no), 尻尾を振る ('wag the tail'), and metaphorically 振られる ('to be dumped' in a romantic context).