To be released from society means to die or to kill yourself.
The children are corrected and scolded for telling some of their playmates that they should be released. It was taken badly by the adults.
The adults know what the term "release" means, but the children do not know. The youngsters do not know what it means, but Jonas finds out what it means when he is learning the job of "keeper of memories". He witnesses his father "releasing" one of two twins. Jonas of course is mortified.
Later in the novel we learn that the current "keeper of memories" intended to pass the job on to his own son, but that son insisted upon being released before the job training was completed.
"What a world, what a world"