Zener diode basically acts as a voltage regulator so it regulates the voltage accross the load where it is connected.
The basic principle behind its working is that it basically manages the current flow accross the load such that life voltage drop accross the load should remain same.
It is basically connected in parallel accross the load. So tgerefore , the voltage rating of the zener will be the regulatory voltage which has to be maintained accross the load. There is just a maximum amd minimum current that can be safetly managed by the zener diode while maintaining zero regulation.
These limits of zener current defines the limits for the voltage regulator.
I f in case there are variations in the sources volatge or the circuit elements values changes, the the current value is varied in the circuit.
This current is divided in the load and its parallel connected zener diode. Now the zener helps in dividing current such that appropriate amount of current should enters the load to maintain healthy required voltage. while the remaining current enters the zener diode.
in this way zener acts a voltage regulator.