。
She knows her limits.
Literal
She [topic-は] her own limits [object-を] is-knowing.
知っている is the resultative ~ている of 知る ('to come to know') — Japanese marks 'know' as the state of having come to know, not as an inherent quality, so the standard form is 知っている rather than bare 知る. 自分の限界を知る is a familiar phrase about personal maturity: the capacity to recognize where one's abilities or stamina end. The plain form gives this a characterizing, almost epigrammatic feel.