a person or thing that can always be used if needed, for example if somebody/something else is not available or if there is an emergency 后备人员；备用物品
Idioms
on standby
ready to do something immediately if needed or asked 随时可以投入行动；处于待命状态；招之即来 The emergency services were put on standby after a bomb warning. 接到炸弹警报后，各应急服务机构进入待命状态。 As the level of the water rose, villages were put on evacuation standby. 随着水位上升，村庄处于疏散待命状态。 The troops are on 24-hour standby (= ready to move at any time of the day or night). 部队待命24小时（接到命令后24小时内可行动）。
ready to travel or go somewhere if a ticket or something that is needed suddenly becomes available 候补座位（如临时有票或具备其他必要条件便可立即出发的）