Here is a circuit showing zener diode used in serial mode and parallel mode.
Zener voltage is 5V. Source voltage varies between 0 and 8 V.
Zener is always used in reverse mode because in direct mode, it has no interest.
Serial mode :
. Source voltage < zener voltage : output voltage = 0
. Source voltage > zener voltage : output voltage = source voltage - zener voltage
Parallel mode (mainly used) :
. Source voltage < zener voltage : output voltage = source voltage
. Source voltage > zener voltage : output voltage = zener voltage
