Overvoltage protection (OVP) ensures that the PSU is providing the correct voltage levels to the PC. If the voltage on any rail becomes too high, OVP will shut the PSU down in order to prevent damage to any parts; too much voltage to a component can damage or destroy the component, which is the reason ESD is a concern when building systems.
These markings indicate that the manufacturer put in the circuitry to protect your PSU from the inevitable problems of brownouts, blackouts, surges, sags, lags, and spikes. Depending on the severity of the electrical anomaly, the PSU may 'fry' itself in a valorous effort to preserve the rest of the rig that it's powering (this is the desirable outcome - a PSU is much more replaceable than a CPU or motherboard, generally).
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