The frequency of an input or carrier sound is modulated by a second (modulator) sound. When the modulating frequency is low and the waveform is simple, such as a Sine Wave, the result is a vibrato. When the modulating frequency is higher, a more complex sound quality is generated.
This modulator usually sound has a frequency that is within the range of human hearing (20Hz – 20kHz). FM synthesis is able to easily create complex sound qualities.