On standby
Definition 1 of 2
Pro Tip 1/3Emergency Readiness
Used to describe people or things that can quickly respond in unexpected situations. 
The fire department keeps a team on standby for rapid response.
Pro Tip 2/3Immediacy Required
Indicates a high level of urgency and the need for a quick reaction when summoned. 
During the storm, utility crews were on standby to restore power.
Pro Tip 3/3Passive Alertness
Refers to a state of being alert without taking action until it becomes necessary. 
The lifeguard is on standby, watching swimmers in case help is needed.