In terms of sound and music, the ADSR envelope describes how sound changes over time. In terms of physics of wave, a general envelope outlines the extreme points (max and mins) of a wave through a smooth curve.
It is obvious to the human ear that when a musical instrument is played, its volume (amplitude) changes over time. For example when a guitar string is plucked, the string vibrates and the initial amplitude is high. After a brief period, the sound amplitude decays. Different musical instruments will have different ADSR envelopes to describe their sound characteristics.
The “Attack” phase of the sound refers to how quickly a sound reaches its maximum amplitude. This is the initial phase of the sound. For most instruments, this period is extremely short (almost instantaneous). The next phase is the “Decay” phase, or the time the note takes to drop to the sustain level. The “sustain” level is generally the longest portion of time, and this is when the amplitude envelope stays relatively constant. The “release” is the period of time the sound takes to go from the sustain level to zero amplitude and is generally short.