This is a CMOS IC (CD4033) based circuit that can be used to detect the presence of mains AC voltage without any electrical contact with the conductor carrying AC current/voltage. Thus it can be used to detect mains AC voltage without removing the insulation from the conductor. Just take it in the vicinity of the conductor and it would detect the presence of AC voltage. If AC voltage is not present, the display would randomly show any digit (0 through 9) permanently.
If the mains supply is available in the conductor, the electric field would be inducted into the sensing probe. Since IC used is CMOS type, its input impedance is extremely high and thus the induced voltage is sufficient to clock the counter IC. Thus display count advances rapidly from 0 to 9 and then repeats itself. This is the indication for the presence of mains supply. Display stops advancing when the unit is taken away from the mains carrying conductor.