The output voltage is equal to the zener diode voltage less 0.7 volts, or ;
Vo = Vz - 0.7 where Vz is the value of the zener diode.
With the 10V zener diode as shown in the diagram the output voltage is about 9.3 Volts DC. The supply voltage used must always be at least a few volts higher than the zener voltage.