expression, godan verb (-su)
straighten one's posture; sit up straight
Used both literally for correcting physical posture and figuratively for adopting a serious, attentive attitude.
先生が入ってくると、生徒たちは姿勢を正した。
When the teacher came in, the students straightened their posture.
面接の前に
Before an interview, it's important to calm down and sit up straight.