Well, dear child, "God is dead" is a very complicated idea that people talk about a lot. It basically means that some people don't believe in God anymore - either because they don't think He exists or because they've lost faith in Him.
Now, let's imagine that you have a toy you love and always play with. But one day, the toy breaks and you can't play with it anymore. You might say that your toy is "dead" because it can't do what it used to do.
In the same way, some people might say that God is "dead" because they feel like He's not doing things that they want, or that He doesn't exist at all. But whether or not you believe in this idea, it's important to respect other people's opinions and beliefs.