In the song "Imagine" by John Lennon it is trying to say to appreciate the things you have in life because you never know when all of those things can be taken away. Things could be worse.
This song relates to The Giver because the parts that talk about the world be different is how Jonas' world is. There is no countries and there is no religion. It is kind of like Jonas wants all of the good things that John Lennon is saying in the song. A sky and everyone living in peace. This is how I think the song relates to The Giver.