What is Imagine by John Lennon about?

Answer

It was about if there wasn't anything separating each one of us (religion, etc.) the world would be a better place.

To put it plainly, Imagine is a plea for world peace and Lennon was simply stating the reasons (or problems) that caused the most grief in the world and asked the listener to "Imagine" what it would be like if these things didn't exist..

Greed ("Imagine no possessions"..)
Religion ("Imagine there's no heaven" "and no religion too"..)
Nationaliasm ("Imagine there's no countries"..) ("Nothing to kill or die for")

Also, it's about the central ideas of Communism, not just anti-socialism.
For a while, Lennon was pro-communism, and obviously against socialism.