Adjustable pressure relief valves, as their name suggests, are mechanisms whose primary objective is to relieve (or perform what we call a by-pass) pressure, but in a fully controlled way. If we talk about a pressure and temperature relief valve, you will also consider the latter.
This mechanism basically consists of a plug that keeps the exhaust opening closed. It also has a spring that has been carefully calibrated and that keeps this plug in the most appropriate position to prevent the contents of the system from escaping.
When the internal pressure of the gas or fluid exceeds the regulated pressure in the spring, the plug is opened to let the contents out through the exhaust opening. This, of course, will only happen as long as the internal pressure remains too high; once it decreases sufficiently, the plug returns to its initial position and the system is completely sealed again.
As we talk about an adjustable pressure relief valve, the pressure threshold that will mark what the release point can be adjusted can be adjusted by the pressure exerted by the spring. Usually, this is achieved by a screw that goes through it and that must be tightened or loosened as desired.