Standby mode turns on automatically once the ignition is turned off. You don’t have to worry about turning it on.
Standby/reserve only mode means that the vehicle is still hired and the booking is active, yet you are not actively using it. You can use this within parking zones so the car is not taken by someone else, or as normal whilst you are outside a parking zone.
The price when on standby mode is $0.10/min.