An ADSR envelope, which stands for attack, decay, sustain and release, is an envelope used to shape synthesized sounds.
Attack time - This is how long it takes from the moment the synth is triggered until it reaches it's peak level.
Decay time - This is how long it takes for the sound to decay to the set sustained level.
Sustain level - This is the level that the note sustains at as long as the synth is triggered.
Release time - Once the key is released, this is how long it takes for the sound to go to zero.