A fuel pressure regulator gauge is a device used to measure and display the fuel pressure in the fuel system of a vehicle, particularly in fuel-injected engines. 
The primary function is to provide real-time readings of the fuel pressure in the fuel rail or system, helping to ensure the engine is receiving the correct amount of fuel.
It assists in diagnosing issues related to fuel delivery, such as fuel pump problems, clogged filters, or faulty regulators. Abnormal pressure readings can indicate potential issues.