The default ideal temperature for is 6°C.
Hibernate mode is the mode to choose for when you want the heating off. Two examples of this would be:
The system will remain in Hibernate until such time as you choose a new mode. Your scheduled heating modes will be suspended so you can be sure that the heating remains off.
If you want the heating off in the winter (perhaps you're off to somewhere sunny and warm), Hibernate will make sure that your home never drops below 6°C so you can be sure that your home is safe from a harsh winter.
You can put your Cosy system into Hibernate mode from the settings screen of the Cosy App, or by pressing the Hibernate button on the display.