Trace the outline of a waveform and you have its envelope: a pluck spikes and dies, a pad breathes in and holds, a swell climbs. Synthesists formalize it as ADSR (attack, decay, sustain, release), the standard four-stage description of shape.
Envelope explains much of why instruments feel the way they do. The same pitch with a violin’s slow bloom or a marimba’s instant decay reads as entirely different gestures.
Visually, envelope is gesture. Following a sound’s envelope with size, brightness, or velocity is how motion inherits the phrasing of the playing rather than just its volume, and an envelope follower is the tool that extracts it live.