The REES RS1900 75W Zener Diodes are a type of electronic component that is designed to regulate voltage in a circuit. It has a Zener voltage of 1900 volts, which means it starts to conduct current in the reverse direction once the voltage across the diode reaches 1900 volts. The REES RS1900 75W Zener Diodes have a maximum power dissipation rating of 75 watts, which means they can handle high levels of current without getting damaged.