What does the rectifier do in a crystal radio?
It's called an envelope detector. The diode prevents the base frequency from going negative. The original signal had an average value of 0. If you fed this through a low-pass filter (aka a capacitor), the output signal would be 0. With the diode in place, the signal can never go negative and now if you average out your signal using a low-pass filter, you get a slowly varying signal (relative to the base frequency) that no longer has an average of 0. This signal is now useful for the speaker.


https://en.wikipedia.org/wiki/Envelope_detector