The voltmeter basically indicates whether the battery is in a 'charging' state or a 'discharging' state.
Generally speaking, in order to charge a battery, input voltage has to be higher than the battery. For example, when the input voltage is higher than 12-volts, a 12-volt car battery will be considered to be in a charging state. If the voltmeter is below 12-volts, that means the battery isn't charging but draining instead. Prolonged 'under' will drain out the battery and you car will die of a weak battery.
Excellent example: Traffic Jam (stand-still traffic) at night with A/C,all lights, brake-lights (full-load in other words) on for a prolonged periods of time will result in weak battery. Voltmeter should be under 12-volts (Unless the generator has a humongous capacity (that generates massive amps.)
The voltmeter actually is best to be used in conjunction with the amp-meter since both complement each other on the status of the electrical system.
Also - a "dead" battery may actually be a weak battery. It may show up as 5v (as an example) on the voltmeter but be unable to turn over the engine or run the car. With a voltmeter at least you can tell that the battery is hooked up and you have some rough idea of the condition (funcitional or not) of the battery.
The voltmeter is useful at monitoring your electrical system. The resolution of the instrument is somewhat useless. Something like 7-16V would be more useful. Anyways with the engine running you should get about 14 volts. With the engine off you should have about 12.5V. Once a battery discharges to 10.5V it has essentially no useful power left. When cranking the battery voltage shouldn't dip below 8.5-9V in the summer and 8-8.5V in the winter. If it does than your battery is about to go. With such a small instrument it might be hard to accurately measure this.