An envelope follower listens to a signal and continuously answers one question: how loud is it right now, smoothed enough to be useful? Its attack setting governs how fast it rises with the sound, its release how gracefully it lets go.
The follower is the workhorse of audio-driven anything: sidechain effects, auto-wahs, ducking, and virtually every parameter in audio-reactive visuals ultimately runs through one. Raw level flickers; the follower turns it into a curve that moves like a hand on a fader.
Tuning is taste made explicit. Fast attack with fast release is nervous; fast attack with long release is punchy and breathing, which is why that combination appears everywhere from compressors to Photism’s reaction controls.