It's a confusing switch for some. It's labeled "BATTERY DISCONNECT". On or off. Maybe I've had too many English classes but battery disconnect ON could mean the battery is disconnected, and vice versa for OFF.
But no, ON means the battery is connected. Usually the switch needs to be on to charge the battery from a 110volt connection or when connected to a tow vehicle. And, OFF isn't really completely OFF. The smoke detector, CO detector, radio lights, etc. will still be on.