Lumen Gentium is a fancy name for something important that happened a long time ago. Just like how your parents make rules to keep you safe and happy, a long time ago, some very important religious leaders made rules for how people who believe in God (specifically, Catholics) should be like.
These rules were written down in a very big book called the "Dogmatic Constitution on the Church." Sounds fancy, doesn't it? It's like a really important rule book that has rules about how Catholics should worship God, how they should behave toward each other, and how they should organize their church.
Lumen Gentium is just another name for this book. It means "Light of the Nations" in English. The people who wrote this book wanted to show that God's love and light can shine through everyone and everything, not just Catholics or people who believe exactly like they do.
One of the most important ideas in Lumen Gentium is that the Church isn't just made up of priests or nuns or important bishops. Everyone who believes in God and follows Jesus is part of the Church. So even regular people like you and me can be a part of this big, important community that loves God and helps others.
Lumen Gentium is a way for Catholics to remember what their religion is all about, and how they can be better people who love God and love others. So the next time someone talks to you about Lumen Gentium, remember that it's just a fancy name for a book that helps good Catholics be even better!