In this circuit, the output voltage is maintained constant by providing constant zener current.
Suppose that the output voltage tries to change, i.e. there is change in the voltage drop across R3 appropriately. Since Voltage across R3 is at the inverting terminal, the output is in the negative direction. Hence, any change in the output voltage is multiplied by the corresponding change at the inverting terminal. Thus the output voltage remains constant.
* The constant output voltage can be obtained by selecting suitable values of R2 and R3.
