A pressure reducing valve lowers the high water pressure coming from the city main to a steady, set pressure before it reaches your pipes, fixtures and water heater. It sits on the main line on your side of the meter, usually close to where the line comes up beside the house, near the main shut-off.
Inside, a spring pushes against a diaphragm. As house pressure rises toward the setting, the valve closes down, and when you open a tap, it opens to let more water through. An adjusting screw changes the setting. It is also a code item: the 2021 International Residential Code, which Plano adopted for homes, caps static pressure at 80 psi and requires this valve where the main pressure is higher.
A worn valve usually fails one of two ways. If it starts letting street pressure through, you may hear banging or noisy pipes, see toilets that refill on their own, or find the relief valve on the water heater dripping. If it sticks or its strainer clogs, pressure drops or swings at every fixture at once. A gauge screwed onto an outdoor hose bib, read with all other water off, shows which problem you have. Code lets some hose bibs be piped ahead of the valve, so a reading there may show street pressure rather than house pressure.
In most homes the valve also closes the system. Water that expands as the heater warms it cannot push back to the main, which is why a home with one needs a thermal expansion tank. The honest limit is that the valve only lowers pressure. It cannot raise it above what the city supplies, and turning it up will not fix low pressure in the whole house caused by a leak or scaled pipe.