I don't use the command and I don't have that file either but I'll give this a try. mknod creates the device nodes for basically every device on the computer. I would think the file in /etc would contain some sort of rules or some other info needed by the command.
I would add, if you don't know about mknod, I would leave that file be. It could be that if you mess up or change the file, even by accident, your system may not boot or may have other problems to pop up.
You may also want to just go to a console and type in "man mknod" and give that a try. It's sort of geeky but sometimes it gets you started.
Hope that helps.