The envelope is used to produce changes in the sound in relation to the time of the keypresses as you play the keyboard. The EGs are the biggest factor in determining the feel of the sound as you play it.
The EG controls are grouped by operator, with operator 1 on top, followed by operators 2, 3, and 4.
For example, the sound of a plucked string like a guitar would have a fast attack rate, the sound of a violin would have a medium attack rate, and the sound of a passing car would have a slow attack rate.
The decay one rate tends to affect the sound of the attack rate. If the decay one rate is fast, it will make the attack sound sharper. For instance, a bell hit with a hard mallet would be emulated with a fast decay one rate, whereas a piano, which doesn't have such a sharp sound in the attack, would be emulated with a slow decay one rate.
If the D1L is set to 15, then the D1R has no effect, since the sound has already "decayed" to 15 as soon as it reaches the attack level, which is fixed at 15.
The decay one level determines how much bite the attack of the sound has. For a marimba sound, for example, you would give it a fast attack rate and decay one rate, the set the decay one level somewhat low. This will give the impression of a hard mallet hitting the bar.
The decay two rate is used for the ringing out of a sound after the character of the attack has played out. For a chime sound, you would use a fairly slow decay two rate, so the sound rings out for awhile as you keep the key pressed.
For example, a typical organ sound turns off immediately when you release the key, so you would use a high release rate. If you wanted to make a chime sound that you could play by quickly hitting the keys as you would hit the chimes, you would use a slow release rate so the sound rings out after you release the key.
This is most useful on modulators for leaving some tone in the sound after the operator has fully decayed to zero. It's pretty much analogous to the L4 level on the DX7.