Well, buddy, the Euston Manifesto is a kind of letter or document that some grown-ups wrote a long time ago. It's like when you draw a picture and then show it to your family, but this time many people wrote it and everyone can read it.
The grown-ups who wrote the Euston Manifesto wanted to talk about some important stuff. They wrote about how they believed that some people in the world were being treated unfairly because of where they came from, what they believed in, or how they looked. They thought this wasn't right and everyone should be treated the same, no matter what.
They also talked about how important it was to be honest and to always say what you really think. They said that people should be able to have different opinions and ideas without getting into fights, and that everyone should listen to each other and try to understand each other.
In short, they wanted to encourage people to be fair, honest, and kind to each other. And that's a really nice thing to do, don't you think?