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.