The fuel pressure regulator is a component that maintains a high enough fuel pressure for the system it supplies, be it a carburetor system (low pressure) or a fuel injection system (high pressure). The regulator allows excess pressure to flow back into the fuel tank via the fuel return line. Some fuel pressure regulators are controlled only by a spring, while others have a vacuum line feed which allows the manifold pressure to influence the fuel pressure.
Fuel Pressure Regulator Failure
Aside from getting clogged up or leaking, there are two mechanical ways a fuel pressure regulator could fail. One way is that the spring or diaphragm gets weak and allows fuel to return to the tank at too low a pressure (this will cause the engine to run lean, due to a lack of fuel). The other way is that the relief valve gets jammed in some way (which is a less common failure) and doesn't allow pressure relief, which will cause some other point on the fuel system to fail as well.