A diode is a detector, have a look at any am receiver even something as basic as a crystal set
google crystal set you cant get a more basic am receiver
from Wiki
the diode (crystal) detector works.
(A) The amplitude modulated radio signal from the tuned circuit. The rapid oscillations are the radio frequency carrier wave. The audio signal (the sound) is contained in the slow variations (modulation) of the size of the waves. This signal cannot be converted to sound by the earphone, because the audio excursions are the same on both sides of the axis, averaging out to zero, which would result in no net motion of the earphone's diaphragm.
(B) The crystal conducts current in only one direction, stripping off the oscillations on one side of the signal, leaving a pulsing direct current whose amplitude does not average zero but varies with the audio signal.
(C) The bypass capacitor smoothes the waveform, removing the radio frequency carrier pulses, leaving the audio signal
dave