That's Attack, Decay, Sustain, Release. The ADSR envelope is just one type of envelope, but it's a popular one that's been used in electronic music for decades. Attack is the period of time after the initial release, it's typically the loudest part of the sound Decay is the phase while you're going from the attack to the sustain – you're "decaying" Êthe volume from this sharp initial phase to the normal volume phase Sustain is the normal phase of the sound. ÊIt is typically less volume than the attack, and go on for an indefinite period of time, or for a specific amount of time Release is the draw down from the sustain period to no sound. A fade out.