pretty much. when the pressure needs to be low, it just opens up and lets the fuel flow through at whatever pressure the pump is making. But when it wants more pressure, like when you step on the throttle, it closes up which makes the pressure build up in the rail.
An adjustable pressure regulator just lets you manually control the size of the exit hole. the smaller the hole, the higher the pressure.

neat little tid bit. if you want to bump your fuel pressure up a little bit, you can take a screw driver or something and dent in the bottom of the stock FPR. that will keep the pressure higher all of the time.