The amp meter shows how much current the battery charger is outputting to the battery. When you turn on the charger, it outputs a high amount of current (electrical power). For example, if you select the 12-amp rate to charge the battery, the charger needle will be closer to the 12 on the right side. As the battery charges, the needle will drop to approximately half, or 6 amps in this case, of the selected amperage rate indicating the battery is fully charged.