I assume it works the same as any other voltmeter - it's monitoring overall system voltage. If the car is off and the key is in the on position, it will be giving you battery voltage. If the car is running it's giving you overall voltage. The only way to get a direct alternator output reading is an ammeter. Normal voltage is usually around 14-15v, and a lower\higher voltage can signal a poorly functioning battery or charging system. The battery and voltage regulator usually buffers the charging system, and a wildly fluctuating readout can signal an issue as well. Its pretty normal for the voltage reading to fluctuate slightly as electrical draws on the system fluctuate - ie, the battery is recharged from starting, or the A/C is cycling on and off, while running the defroster, etc..