The ADSR (Attack Decay Sustain Release) node controls the envelope of a note. The control signal can be used for the note's volume, or to vary some other parameter such as a filter cutoff.
The envelope starts when the velocity input becomes greater than zero and decays when the velocity returns to zero. The envelope goes through four stages of attack, decay, sustain, and release each controlled by a knob.
The Attack knob specifies how long in seconds until the note reaches full volume.
The Decay knob specifies how long until (in seconds) the note fades to the sustain level which is, in turn, specified by the Sustain knob.
The Release knob specifies how long (in seconds) until the note fades after the velocity returns to zero.