So, imagine you're playing with your toys and suddenly you see something dangerous nearby, like a hot stove. Your mom tells you to stay away from it because it could hurt you right then and there. That's an example of a clear and present danger.
When grown-ups talk about "clear and present danger," they usually mean something that could be really dangerous to a lot of people. It could be something like a disease that can spread easily, or even like a bad guy who might hurt a lot of people.
The government has to be careful about what they do when there's a clear and present danger. They might need to take away some of your freedoms to keep you safe. But it's a hard decision because they don't want to take away too many freedoms and they want to make sure they're really keeping everyone safe. So, in short, clear and present danger is when something dangerous is happening right now and grown-ups have to decide how to keep everyone safe.