Modulation is the technique of varying parameters to sound generators and sound modifiers over time. Modulation is important, because without change over time music is boring. We can modulate just about any parameter, just as frequency, amplitude, phase, copies of the source, and so on.
We can even modulate the entire signal, such as in the case of ring modulation. In ring modulation, a source signal is multiplied by another, simple signal such as a sine wave. This effect can produce dramatic results.