It's a 'fuel pressure regulator' and it's there to do what it says on the can.

The fuel injectors need to supply fuel at a constant pressure and the fuel pressure regulator is there to ensure that constancy. A spring inside pushes against the diaphragm and helps to regulate the pressure inside it. The tension (pressure) of the spring is pre-set to get the desired fuel pressure i.e. so that the fuel pump has to pump enough pressure and enough fuel at the same time to overcome the spring tension; any additional fuel that is not needed is sent back to the fuel tank through the fuel return line.