Jonas is an 11 year who seems to live in a perfect, controlled world. Jobs, spouses, and children are all chosen for them by "elders" who are the wise, old people of the "community".
At the ceremony of the twelve, people are given their jobs. Jonas got a job that he and the other twelves haven't heard of before, the "Receiver of Memory". The cheif elder didn't say much about it, only that he would be in unimaginable pain. There was something special about Jonas.
Now that Jonas is the Receiver, he needs to go through his training. His job is to hold all the memories of the world for his community, and the Giver, or the last Receiver, is giving him all of the memories for his training. It can be pleasant when nice memories are passed, but excruciating when memories of injuries, war, or even death are passed.
Jonas has finally realized what a free, non-controlled world feels like. Feelings like love, happiness, pain and sadness isn't known by anyone in the community. Now he feels that it's wrong, life is about the choices you make which makes you, YOU.
Jonas and The Giver devised a plan. They wanted to change how things were, bring the people back from "sameness" and into a normal life. The memory's you hold get released when you leave the community, or die. Jonas had to use a bicycle to escape the community, so the memory's will be given to others. He will never be able to come back, but he accepts that.