Okay kiddo, do you know what invisible means? It means you can't see it. So an invisible man is a man who you can't see with your eyes because he's not visible.
Now, do you know how we can see things? With our eyes, right? Our eyes see because they can detect light. The light bounces off of objects and enters our eyes, which sends signals to our brain to tell us what we're seeing.
But what happens if the invisible man doesn't reflect any light? That means the light can't bounce off of him and enter our eyes, so we can't see him.
It's like when you turn off the lights in a room and it's really dark. You can't see anything because there's no light for your eyes to detect.
That's why the invisible man can move around without anyone knowing because he can't be seen. But just because he's invisible doesn't mean he's not there. He's still a person, he still has a body and can touch things, he's just really really hard to spot.
Does that make sense, little one?