。
She sat down on the bench.
Literal
She [topic-は] bench [on-に] sat.
座る ('to sit') takes に for the place where one sits — に marks the destination/location of arrival. ベンチ is a Meiji-era loanword from English. Plain past 座った reports the act of sitting down as a single completed event, distinct from 座っている (the resulting state of being seated).