Assuming typical diode forward voltage drop of 0.6 volts...
Any voltage higher than Vcc+0.6 volts (e.g. 2000 volts electrostatic charge) will flow through the top diode to the power supply, before any damage can be done to the inverter circuit, hopefully.
Same for any voltage less than GND-0.6 volts, where the other diode will conduct.
Diode only allows current flow in one direction, so in normal use the diodes will not conduct.
It's limited to the power supply rails plus a diode drop. For example, if the power supply is 5.0 volts, then the input is limited from -0.6 volts to +5.6 volts.