An envelope shapes volume over time. Four knobs, one classic curve, and every note in your rack goes from drone to pluck to pad.
An envelope shapes how a note sounds over time. It is a short CV signal that rises when a gate arrives and falls when the gate ends.
The classic envelope is ADSR: Attack (how fast it rises), Decay (how fast it falls to the sustain level), Sustain (the held level), Release (how fast it falls to zero once the gate ends).
The Clock is triggering the ADSR. The ADSR is doing nothing yet - its output is not patched. Send ADSR ENV into VCA CV and every clock pulse becomes a pluck.