Being on duty; taking a turn to perform assigned duties such as guard or watch. Also used as a verb form (上番する).
今日は夜の上番だから遅刻できない。
I'm on the night shift today, so I can't be late.
交代で上番し、昼間は別の作業を行う。
They take turns being on duty and do other tasks during the day.
警備員が上番して入口を見守っている。
A guard is on duty watching the entrance.