A SPD - Surge Protector Device or some other names used like Spike Suppressor, Surge Suppressor and Surge Diverters are devices designed to protect electrical equipment from voltage spikes / surges of power.
A voltage spike / surge is a unwanted transient surge of power event, typically lasting 1 to 30 microseconds, that may reach over 1,000 volts.
Lightning that hits a power line or even ground can give many thousands, sometimes 100,000 or more volts.
A motor when switched off can generate a spike of 1,000 or more volts.
Spikes can degrade wiring insulation and destroy electronic devices like battery chargers, modems and TVs etc
A surge protector attempts to limit the voltage supplied to an electric device by either blocking or shorting current to reduce the voltage below a safe threshold. Blocking is done by using inductors which inhibit a sudden change in current.
Shorting is done by spark gaps, discharge tubes, zener-type semiconductors and Metal Oxide Varistors (MOVs), all of which begin to conduct current once a certain voltage threshold is reached, or by capacitors which inhibit a sudden change in voltage. Some surge protectors will use multiple technologies.