Plug an oscillator straight into the input of a vca,
Plug the output of a vca to your mixer or whatever you use to hear stuff
Now turn the gain to zero on your vca - you can’t hear your oscillator anymore
Now turn the cv to zero
Plug a sine wave lfo into the cv input and slowly raise the cv value from 0 -10
Can you hear what is happening to your oscillator volume ?
You can plug anything into the cv input, like envelopes, lfo, or even the outputs of other vca’s. I like plugging sequencers into them, which is why I have a lot of sequencers in my racks
There are lots of other uses but this is the most frequent use and it’s when you say that you are using the cv input of the vca to shape the amplitude of a sound or for us simpletons, we can just say volume.
Hope this helps