AM (amplitude modulation) radio signals carry information by varying the amplitude of a radio-frequency carrier wave in step with an audio waveform such as speech or music, while the carrier frequency stays fixed. A receiver detects those amplitude changes and reconstructs the original sound.
According to Encyclopaedia Britannica, amplitude modulation is the variation of a carrier wave’s amplitude in accordance with the characteristics of a signal composed of audio-frequency waves. AM broadcast services still use this method on medium-wave and other bands, alongside FM and digital radio formats.