What does air leak in the fuel system mean?

Answer 1

If the fuel system has a leak it would be leaking fuel. There should not be air in the fuel system unless the fuel tank is empty.

If you got a code saying that, Your car might have a loose gas cap, the test pump is bad, fuel sending unit gasket or a problem with the tank vent valve(depending on make/model). Car now a days have to have an EVAP system to check that the fuel system isn't leaking fuel vapor into leaking fumes(HCs) into the atmosphere.

I would remove/reinstall the gas cap and clear the fault and rerun the test. If it still is showing a code. Again depending on make/model, there might be a pump self test in the scan tool or you would need a smoke machine to find the leak.