Zener diode is a semiconductor device consisting of a P-N junction which conducts the current in reverse direction when a certain specified voltage is reached. It has a reverse breakdown voltage which means it can continuously operate in a reverse-bias mode without getting damaged.
Hence Zener diode can act as voltage regulator, when they are reverse biased. When the input voltage fluctuates or whenever the voltage in the output changes due to variations in the load, Zener diode prevents the load from getting damaged.