Modulation, in general electronics, means the altering of signal strength. In audio devices, modulation is a control feature that varies the strength of some effect over time to alter tonal properties. Some modulation effects mix ("modulate") an instrument's audio signal with a signal generated by the effect called a carrier wave.[70] Other modulation effects split an instrument's audio signal in two, altering one portion of the signal and mixing it with the unaltered portion.
source: Wikipedia