上班 (shàng bān) means 'to go to work' or 'to start one's work shift' — it focuses on the act of being at your job or starting your workday. It contrasts with 下班 (xià bān, 'to finish work / get off work'), which is its natural opposite. 值班 (zhí bān) is more specific: 'to be on duty' for a shift, often in a service or security role.