Linda, that is exactly right. A diode is a semiconductor which only allows positive current flow in one direction. An isolator is a device with two back to back diodes built in, and the alternator is the source of this ‘positive’ current, supplying current when the engine runs. The alternator connects to the isolator junction of both diodes and supplies charging current through each diode, one to the house battery charging system and the other to the vehicle battery. When the alternator is not supplying current (when the vehicle engine is not running) then the house battery can’t short to the vehicle battery and visa-vera due to their respective one-way diodes in the isolator. It means you can turn off the engine safely with each system, vehicle and house, remaining separate.