Jonas lives in a community where everything is carefully regulated and all choices are made for people. They are told when to sleep, what to eat, when to exercise, even who to marry. The people are kept in a state of near childishness where bad things are kept from them. They are married but not really 'married.' They do not have their own children but ones are chosen for them to raise. They have little attachment to each other. The smooth running of the community is everything and anything unpredictable could upset the balance of things. Even strong emotions.
Jonas is becoming a Twelve, which means his career is being chosen for him. Everyone is shocked when he is picked to be the next Receiver: a complicated position which requires one person remembering the history of all the world, including the painful stuff, and keep it from the other people. It's a bit of a mind bender but you'll get it. The present Receiver is now the Giver because he gives Jonas his memories. Of course Jonas's mind is opened to a whole new way of thinking and feeling. A way that his family and friends can't relate to. You can only imagine what this does to him.
The Giver is what Brave New World could have been if it was good. The worlds are similar, except for the fetus tinkering. Love and bonding are suppressed by drugs. Life is mapped out for the people. I can see why The Giver is aimed at kids just entering their teens. At this point in their lives, they are becoming separate from their parents and making more and more decisions for themselves. The Giver teaches them that yes, making decisions isn't always easy and you will make mistakes but the alternative is... this. Also, there are a lot of bad things in the world but it is better to face them than hide from them.
The ending is ambiguous but, when you think about it, perfect. How better to prove the point of the story by making the reader choose the ending?
I was surprised to read that The Giver has been challenged and called "the suicide book." This clearly is a case of 'context is everything.' A society that chooses suicide as a solution to problems is a doomed one. Anyone who read The Giver can draw that conclusion.