The ACamshaft position bank refers to the camshaft position sensor on Bank 1. Bank 1 is the side of the engine block containing cylinder number one. Camshaft A refers to the intake camshaft on that bank. The sensor reads teeth on the camshaft to tell the computer its exact angle.
The ECU uses this angle to control fuel injection and ignition spark timing. If the sensor fails, the engine will crank for a long time before starting. The computer will run on default tables, reducing fuel mileage.
Replacing a bad sensor is usually easy. It is held by a single bolt on the back of the cylinder head. Always grease the new sensor's rubber O-ring before installing it to prevent oil leaks.