ADSR stands for Attack
(time). These are the four components of an "industry-standard
When a note-on triggers an envelope, the envelope enters its attack stage. Over the time specified by the "attack time" parameter, the envelope output increases to the maximum level.
Once the attack time is up, the envelope begins to decay. Over the time specified by the "decay time" parameter, the output decreases to the level specified by the "sustain level" parameter.
The output remains at the sustain level until a note-off is sent to the envelope, at which point it begins to decrease to zero. By the time the Release time is up, the envelope has reached zero.
Real world examples: a pianoforte's volume envelope would have a very short attack, a long decay, zero sustain, and a short release. An organ's would have very short attack, decay and release values, and a very high sustain level.
So what happens when an envelope is interrupted in mid-stage? Each stage just goes about its business. If the attack is triggered in the middle of a release, the envelope increases from where it left off. If a note-off is encountered during the attack or decay stages, it begins decreasing at the rate required to meet the release time. This is an advantage that parameterized envelopes such as ADSR have over graphical envelopes, which would likely just suddenly skip steps and jump from one value to another.