If you imagine the world that we live in like the world John Lennon is telling us to imagine it like, its perfect. The world that we live in is cruel, full of hate, racism, murder, and war. The world that John Lennon sings is tells us how we should want the world to be. In his song there is world peace, and everyone is living together as one people sharing the world. It's realistic, overall something people need to realize. It covers violence issue and I don't understand while people judge, this will go into that also, it's more than just a songs it's a meaningful poem about realistic life things.
The lyrics say to imagine a world where our differences - religion, nationality, social class - do not divide the human race into warring factions. There may or may not be a heaven or hell, and I doubt John expected everyone to trash their religion, country and possessions, but just imagine for one moment that those things didn't come between people. That instead of presidents and dictators deciding policy, diplomacy was achieved by a fruit vender on the streets of Tehran (Iran) and a window washer in Manhattan (US). No one would be building bombs, flinging around rhetoric in the news and announcing to the world that war is only six months away.
the borders of the countries are there in real life, but in the dreams, they can be omitted. Nobody has any material possessions that they call "theirs", everything belongs to everyone. No one is greedy or hungry for power. it's so pleasant to imagine that everyone is sharing the world, instead of fighting for the imaginary boundaries. All these fantasies can never come true, so he's a dreamer, he knows that there are many other people who dream of peace, love and equality like him. He hopes that someday, everyone will have the same dream, and it won't be a dream anymore. It will become reality. Then we won't be separated by our countries, or beliefs and we'll live together as one.