As Mazz said, a envelope follower generates continuous CV from a signal you put into it by measuring its voltage. A common application is ducking or side chaining where one sound temporarily suppresses another to keep maximum presence. Like a kick supressing a bass line or a voice pushing back background music in a video.
Something like this:
ducking sound --> env follower --> inverter --> vca controlling ducked sound
-- wiggler55550
This is super usefult, thanks: "ducking sound --> env follower --> inverter --> vca controlling ducked sound"