The moral lesson of The Giver is that people must remain free to live their lives as they see fit, without being subjected to control by an authoritarian governing body. Although this futuristic society has successfully eliminated hatred, pain, prejudice, and a variety of other worldly ills, it has done so at tremendous cost.
In the world depicted in the novel, any true sense of humanity has been removed from society, along with all collective memory of anything beyond the community. This burden of memory, with all the joy, pain, hope, and anguish that it brings, is placed on the shoulders of one individual, who is the Giver.
It is clear that an abundance of inalienable human rights are being violated in the community. For example, newborn children are taken away from their mothers and spend the first year of their lives in a care center. Even more disturbingly, citizens who break laws, together with elderly people and children born with birth defects, are "released," which is a euphemism for euthanasia.
Readers are introduced to this world through the eyes of Jonas, who is about to about to become the new Receiver of Memory. It is perhaps this job more than anything that proves how morally skewed the community is.