The song Imagine by John Lennon is about the whole world coming together as one. There is no religion or countries, just a world. A world where everyone lived peacefully and no one was fighting.
It relates to The Giver because Jonas' community they're all the same. No religion, no jealousy, no hate, no fighting. Just peace. Everyone gets along and lives happily. Jonas' world takes a turn when he was chosen to be the next Receiver of Memory, he later finds out that his community isn't all that perfect. Like before his community switched to sameness there was color, there was snow, and the sun and moon, there was jealousy, and fighting. There are higher people controlling his community and hiding secrets that no one knows of.