The charge light is connected between +15 (ignition on) and the regulator. This circuit provides a path from the battery to the field coil to provide a magnetic field to the rotor that will start the alternator charging when it is turning. Until the alternator is running, the electromagnet gets its power from the battery (trough the control lamp). When the lamp is glowing, it means that the alternator "field coil" (electromagnet) is being powered by the battery. Once the alternator kicks in, the lamp goes out because there no longer a voltage difference.
see also:
https://www.kroonwireharnesses.com/other-items?product_id=1294