Blasphemy against the Holy Ghost is a big and confusing topic, but let's try to explain it so that even a five-year-old can understand it.
The Holy Ghost is a special part of God that some people believe lives inside of them after they are baptized. People who believe in the Holy Ghost think that it helps them to be good and do the right thing.
Blasphemy is when you say or do something bad or disrespectful about God or something holy. For example, if you said a mean word about your teacher or your grandma, that would be disrespectful. If you said something bad about God, that would be blasphemy.
So, when people talk about blasphemy against the Holy Ghost, they mean saying or doing something that is really, really bad or disrespectful about the Holy Ghost. And it's bad because some people believe that if you commit blasphemy against the Holy Ghost, you might not be able to be forgiven for it.
But remember, this is just what some people believe, and there might be different ideas about what blasphemy against the Holy Ghost really means.
Overall, it's important to be kind and respectful to everyone, including things that other people think are holy or special, like the Holy Ghost.