waiting room; standby room; duty room
A room where people wait on standby, such as for work shifts, performances, or appointments. Similar to 待機室.
See also: 待機室
The performers are waiting for their turn in the standby room.
待機室 is a more common synonym with the same meaning; 待機部屋 uses 部屋 (room) and may sound slightly more casual or specific to a particular room.
Compound of 待機 (standby, waiting) and 部屋 (room).