Explanation of the Concepts

'Modulation' is the process of varying some parameter of a periodic signal by another periodic signal. 'Amplitude modulation', consequently, refers to the process of repetitively varying the amplitude of some signal over time. For an audio signal, the amplitude is proportional to the volume of the sound, so modulating the amplitude just means continuously turning the volume up and down. Turning it up and down at a sub-audio rate can create many interesting effects, including stereo and quadrophonic "orbit" panning. Turning the volume up and down faster than the audio-rate can produce new, interesting and often very non-harmonic timbres.