Zener diodes are used in the "REVERSE" polarization mode, i.e. the anode connects to the negative supply, and its curve characteristics IV above, we can see that the Zener diode has a region in its reverse bias of almost constant voltage characteristics regardless of the current flowing through the diode. The voltage at the terminals of the diode (it is Zener voltage, Vz) remains roughly constant, even with large variations in current through the diode caused by variations in the voltage of power / charge. This ability to control itself can be used wisely to regulate or stabilize a source voltage against supply or load variations. The led will continue to regulate until the diode current is less than the minimum value Iz in reverse breakdown region.