A voltmeter measures electrical pressure, that's what the term volts describes. It's not unusual for a voltmeter to show less than 12V with the engine not running and loaded, by that I mean with stuff light the ignition energized, lights on, etc. Any load will pull the voltage reading down. With the engine running, the alternator should be able to power all the loads as well as return some juice to the battery and the reading should be in the 12.5-14.5V range. If the battery is run down, the reading may have a hard time reaching 12V until the battery gets charged up.
Amperage is the work or heat component of electricity. When the amperage, also called current, load goes up, like a teeter totter, the voltage goes down. The location of where the meter taps into the circuit makes a difference in the "normal" running voltage. If it's located at the very end with stuff like the headlight and running lights between it and the alternator, then the reading is going to be lower than if it taps into the wire immediately coming out of the regulator. Also, meters can vary somewhat in accuracy.
As mentioned, note the readings it shows and if that suddenly changes or is low and never increases regardless of battery charge or load, then you may have a charging problem.