Meaning of “standby” - Learner’s Dictionary

standby

noun [ C ] uk us /ˈstændbaɪ/ (plural standbys)

someone or something extra that is ready to be used if needed:

We kept our old TV set as a standby in case the new one broke.
be on standby

to be ready to do something or to be used if needed:

Police were on standby in case there was any trouble after the game.

(Definition of “standby” from the Cambridge Learner’s Dictionary © Cambridge University Press)