An Ammeter indicates which power source is powering your car, either alternator or battery.
The regulator is there to Command the alternator to maintain a slightly higher output-pressure (voltage) than the battery.
If the ammeter is bouncing on the switch-point, it could simply be because the idle-speed is too low. But if it continues to bounce as the rpm increases, something may be wrong.
The Regulator has the primary job of sensing battery voltage, and maintaining the battery at a predetermined voltage. It does this by turning the field coil on, and checking the voltage again. When the voltage goes over it's set point, it shuts the field coil off, and checks the voltage again. Then when the voltage falls, it turns the field coil on again. It does this continuously at a high rate of speed, and so a sensitive ammeter will show the handiwork of the regulator.
Usually the ammeter is electrically damped, and kindof a slouch. But in your case it seems not to be damped at all. No big deal; I think.
But to be sure, you should do an Amp-Draw test, to rule out an inordinate power draw, or a short, somewhere in the system. The heater is often a major player, with the headlight circuit coming in a close second.