We don't find out what, exactly, until the end, as no one wants to talk about it. I was so suspicious of when Jonas left because it seemed so easy, but on the other hand, there's no reason in the world to expect that anyone would want to get away from this world.
In this answer I'd just like to add some some quotes where Lois Lowry directly confirms that she has indeed intended to write an ambiguous ending.
It then continues while Jonas the main character waits to receive his assignment in the community. The climax is when the plan to escape the society is being conceived and the actual escape itself.
You think, He can't go back to her, there are still these guys after him. But now, reading the book as an adult, it is harder to interpret the ending. I read it in one sitting; I got two pages in and moved my towel under the umbrella and finished it there.
Kate and I both read The Giver and then talked about what we think it means, why it's an important part of the kids' book canon, how it impacted books that came after, and how it felt to read it as adults. So I'm always kind of surprised and disappointed when some people tell me that they think that the boy and the baby just die.
The second that you start saying someone's going to be "released"—they talk about sending off the old dude, and it's clearly a funeral with the guy present.