envelopes
introduction
The envelope describes the shape of the change in amplitude over time.
text
definition and some history: https://en.wikipedia.org/wiki/Envelope_(music)
more detail about envelopes: https://www.pitchbends.com/posts/sound-envelopes-adsr
video
basic and great explanation with examples: https://www.youtube.com/watch?v=Q-ot9AaJx-Y