Pickups are devices which convert the mechanical motion of a guitar string into an electrical signal. They do so by using magnets and coiled wire. When a string moves back and forth in the magnetic field above the pickup, an electrical signal is induced into the pickup coil.
That electrical signal is proportionate to the amount of movement of the string.
Since the vibrations of the string also are influenced by any overtones which are added by the other components of the pedal steel guitar, the total shape of the string vibration will reflect those influences, as well. All of that again will be transmitted as an electrical signal to the volume pedal and then to your amplifier. For more detailed information, most any physics book or magnetic theory book will explain exactly how an electrical signal can be induced into a coil by crossing a magnetic field with a piece of wire.