Blocks used for modulation produce slowly changing signals that cannot be heard directly. These modulation signals are called CV (Control Voltage) on analog modular synthesizers.
When you apply modulation to controls in your patch, the controls change their value according to the shape of the modulation signal.
The most basic building block for modulation in synthesizers is the envelope. When triggered by a note event, the envelope produces a single contour that can be used to change controls over the course of a note.
A common type of envelope is the ADSR, named after the stages of its contour (Attack, Decay, Sustain, Release). In the subtractive synthesizer you can use it to control the volume level of the sound, for example. This is done by applying the modulation from the envelope to the VCA.
