Pop-Off pressure is the pressure at which the needle valve will open to let the fuel to enter the metering chamber. That's the short answer. The pop-off pressure required depends on the carburetor and engine.
If the pop-off pressure is too high, it can lead to fuel starvation especially at high RPM, which leads to overheating and possibly seizure.
If the pop-off pressure is too low, the carburetor will flood, especially at low and mid RPM, making it difficult to restart a warm engine.
Every carburetor has a different pop-off pressure setting. Some carburetors on new paramotors are not set correctly. In fact, one manufacturer didn't even know what the correct pop-off pressure should be on their own engine!
The majority of paramotors will have a pop-off pressure around 10 psi to 12 psi. You can build a pop-off pressure gauge with an automotive cooling system pressure tester along with a couple of extra parts.