The zener diode breakdown at a fixed voltage and independent
of current in it, it maintains that Vz across it.
In the forward direction looks like a "normal" diode, but in reverse direction,
when it breaks down, it maintains an approximate constant V independent
of current thru it. So if a load is connected in parallel with it, it will maintain
a constant V on the load independent of load changes, relatively speaking.
The R is to limit Iz so that the zener does not burn out. Limits current at max
Vinresistor.
Regards, Dana.