A device file is like a toy box with many different toys in it. Each toy is a device that the computer uses to do different things. For example, the mouse is a device that helps you move the cursor around the screen.
The toy box is located inside the computer and has a special name like "/dev". When you want to use a device, you tell the computer which toy to use by calling its name. For example, you might say "use the mouse toy to move the cursor."
The computer uses device files to make this communication easy. Each toy has its own file in the toy box, and the computer knows exactly which file to use when you call it. That way, you don't have to remember the name of every individual toy - the computer takes care of it for you.
In short, device files are files that represent different devices inside the computer that help you do different things, and they make it easy for the computer to know which device you want to use.