When creating artificial waveforms in a synthesizer, changes in the signal amplitude (or frequency) over time are controlled by an ‘envelope generator’ which typically has controls to adjust the Attack, Decay, Sustain and Release times, triggered by the pressing and subsequent release of a key on the keyboard.
A standard ADSR envelope.
The Attack phase determines the time taken for the signal to grow to its maximum amplitude, initiated by the pressing of a key. After the attack phase the envelope then immediately enters the Decay phase during which time the signal level reduces until it reaches the Sustain level set by the user. The signal remains at this level until the key is released, at which point the Release phase is entered and the signal level reduces back to zero.
Some systems use a simpler attack/release system, or add extra facilities like delay and hold time controls, and Yamaha came up with a format that removed the sustain control but introduced 'Initial' and 'After' (IL and AL) level controls for the attack phase, effectively setting the range of the attack portion below and above a notional sustain level.