Okay kiddo, so the Gospel of Marcion is a very old book from a long time ago, that people used to learn about Jesus and his teachings. But not everyone thought it was a good book to have.
You see, people back then had different ideas about what Jesus was all about. Some thought he was all about love and kindness, while others thought he was all about rules and punishment. Marcion was one of those people who had a different idea about Jesus.
Marcion believed that there were actually two different gods in the world, one who was loving and forgiving, and another who was angry and punishing. He thought that the god of the Old Testament was the angry one, and that Jesus came to tell us about the loving one.
So the Gospel of Marcion was written to share this idea about Jesus. It talks a lot about how he was sent by the loving god to save us from the angry god. But not everyone agreed with this idea, and some people thought it was actually kind of dangerous to believe in.
In fact, eventually most people decided that the Gospel of Marcion wasn't a good book to have around at all, and they stopped reading it. But even though it's not used much anymore, it's still an important part of history and helps us understand how people used to think about Jesus a long time ago.