ADSR is an acronym for attack, decay, sustain, and release. These elements define the sounds, or envelope, produced by the envelope generator of a keyboard instrument or synthesizer; they match the physical attributes of naturally occurring sound.
The attack, decay, and release parameters are rate or time controls. Sustain is a level. When a key is pressed, the envelope generator will begin to rise to its full level at the rate set by the attack parameter, upon reaching peak level it will begin to fall at the rate set by the decay parameter to the level set by the sustain control. The envelope will remain at the sustain level as long as the key is held down. When a key is released, it will return to zero at the rate set by the release parameter.