A Mii binary file is a file downloaded to your computer from a Wii remote that represents a Mii character. This file contains the data that the Wii remote uses to transfer Miis to and from Wii consoles. You can download these files from your Wii remote to your computer with applications like the Mii Mega-mall. Once on your computer you can load them into the Mii Editor on
http://www.miieditor.com to view and edit. The Mii Editor can also create Mii binarys from scratch which you can then load on to your Wii remote as new characters.