What does the release of a contributing citizen represent?

Basically, the release of a contributing citizen represents death. In the story, we are first introduced to the notion of 'release' in Chapter One, when a pilot-in-training flies too low over the community. Accordingly, the pilot had 'misread his navigational instructions and made a wrong turn.'

For punishment, the voice over the loudspeakers proclaims that the offending pilot will be released.

For a contributing citizen to be released from the community was a final decision, a terrible punishment, an overwhelming statement of failure.

In Chapters 18-20, we finally discover what 'release' means. The Giver admits that he thinks about release when he is in a lot of pain. However, he tells Jonas that he cannot be released until a new Receiver has been trained. In Chapter 18, The Giver admits that the last Receiver, Rosemary, had requested her own release. All Jonas knows at this point is that the Giver was greatly saddened and affected by Rosemary's release.

In Chapter 19, Jonas watches his own father release one of the twin Newchildren. Initially, Jonas thinks that a release is a special ceremony of sorts. So, he isn't especially worried about the baby's safety and is only mildly curious when his father takes out a syringe, fills it with a clear liquid, and injects the baby's forehead. However, Jonas starts to panic when the child eventually stops crying and moving his limbs. To his horror, he recognizes that the blank look on the Newchild's face almost directly mirrors that on a dead soldier's face he once saw. Numb with fear and anguish, Jonas now understands that release means death.

His horror is compounded by the fact that he saw his own father nonchalantly place the Newchild's corpse into a box before shoving it down a waste disposal chute. After seeing this, he also comes to realize what Rosemary's own release had meant: she had basically committed suicide.