"Standby mode" is a common phrase used to describe a device or system that is in a state of being ready to be used or activated, but is not currently in use. This can also be referred to as "idle mode" or "sleep mode." For example, you might say "I put my phone in standby mode when I'm not using it to save battery" or "My computer goes into standby mode when I leave it idle for a long period of time."
