A VSR allows you to connect two batteries together for charging purposes, while keeping the rest of the system completely isolated.
When your vehicles engine is running and the main starter battery is fully charged (13.3V) this VSR will kick into gear and charge the second or auxillary battery. Then, once the engine is switched off and the starter battery voltage drops to 12.8V, the VSR will disengage and thus prevent the starter battery from being drained while your auxillary battery powers your appliances.
 
This VSR is suitable for marine, off roading, and solar setups for caravans.