Okay kiddo, have you ever heard of a new name before? It's when we give something a name because it doesn't have one yet. We do this a lot with things we discover in science like animals or plants.
Well, nomen novum is just a fancy way of saying "new name" in Latin. Scientists use this term when they discover that something already has a name but it's not quite right. Maybe the old name is confusing or misleading, or maybe it's just not accurate anymore.
So, they come up with a new name that fits better and use nomen novum to let everyone know that this is the new official name. It's important to have clear and accurate names for things so that scientists and other people can all talk about the same thing and understand each other.