Answer:
"am" on a radio dial refers to "amplitude modulation". The signal (voice, music) is coded onto the radio wave by changes in intensity. Somewhat like sending a message by turning a light dimmer up and down to change the brightness.
The other radio band, "fm", stands for "frequency modulation". The signal is coded onto
the radio wave by changes in frequency. Like sending a message by changing the colour of a light from red to blue.