A safety valve is a pressurised valve that opens in case of overpressure and protects the rest of the system. If the pressure in the system rises, the safety valve spring compresses, causing the valve to open and release the excess pressure to remove the risk of explosion and pressure damage.
The design of the valve allows it to close again afterwards to prevent further release of the medium once the pressure ratio has returned to normal. Safety valves should be used where pressurised equipment is present.