The term "envelope" comes from the synthesizer school system of using envelopes to shape not only frequency control amounts but also volume.
An analog synthesizer like a Minimoog will show you an ADSR (Attach Decay Sustain Release) which will create envelopes.
There will be an ADSR control on the Filter and the Amplifier.
Here is the filter and amplifier section of a similar monophonic analog synthesizer made by Sequential Circuits, the Pro One.
In the above photo you can see there is a control for the amount of envelope you desire along with the amount of keyboard control.
The
resonance is the filter frequency selector and the
cutoff is like a tone control which allows how much of the frequency above or below a threshold of upper and lower frequencies to get through.
One would be best served by experimenting with an actual synthesizer to get the idea.
The ADSR creates how much of an attack, how long before it decays, how long it will sustain and how long before the sound is released and dies on both the filtering and the volume of the sound.
A slow attack will, for the filter, make the wah sound open gradually rather than abruptly as with a fast attack. So you get a "buuuuwaaaaahh" sound on a slow attack and "buwah" on a fast attack.
The envelope filter pedals, the amount of attack you put through it with your guitar will cause the filter to increase the filter's effect. If you pluck harder the filter will respond harder. Hence it is a matter of learning to "play the filter pedal" not just play your instrument through it.
The auto wah has a set envelope which makes it react like a normal wah with a sweep pedal like a Crybaby wah. The envelope pedal will react to how much input is run through it so you will get wild responses depending on how you pick your guitar.
Many get confused and think the envelope pedal is junk when using one is something that has to be understood.
Bootsy used knock offs of the Mutron to get "trashy" sounds out of his rig.
The Mutron effect perhaps you've heard it through Stevie Wonder's clavinet on
Higher Ground is an envelope filter.
Auto wahs below.
Below, the T wah is a "touch wah" or an envelope filter and the envelope will respond on how you pick your guitar.
DOD has this envelope filter that many use.
An envelope filter pedal has more of an unpredictable response where as an auto wah is more controlled.
The use of your volume pedal on your PSG is you creating an
envelope for volume control.
