。
She is brushing her hair.
Literal
She [topic-は] hair [on-に] brush [object-を] is-applying.
ブラシをかける is a stock idiom: 'apply a brush to' — the everyday way to say 'brush.' ブラシ is a loanword from English 'brush.' The に on 髪 marks the surface to which the brush is applied.