An input into part of the synthesizer that allows that bit to be modulated by another part of the synth. For example there is a control input in the amplifier which controls the level of amplification. If a very slow sine wave (an LFO) is patched into this input then the sound will slowly get louder and quieter. If an LFO was patched into the control input of the oscillator instead, then the sound would go higher and lower in pitch....
