The purpose of an overvoltage protection is to protect the electrical equipment in case of overvoltage. Overvoltage can be caused by lightning, capacitive current or inductive coupling of other electric systems.
Wikipedia definition (similar term):
When the voltage in a circuit or part of it is raised above its upper design limit, this is known as overvoltage. The conditions may be hazardous. Depending on its duration, the overvoltage event can be transient—a voltage spike—or permanent, leading to a power surge.
Source: Wikipedia - Overvoltage