This file is provided as a way to list the output of a blog without including the layout and styles of a skin. This will produce a very plain looking page with your blog posts, comments and other content elements, which can then be used as the user sees fit.
`a_noskin.php` no longer ships with b2evolution v6+. However, you can download it from this page (just change the extension from `.txt` to `.php`).
Place this file into your base directory (the /blogs directory in the installation package).
If you open up `a_noskin.php` in a text editor, each section of the file is commented to let you know how to configure the file correctly. As the file comments say, "You only need to use this file for advanced use/customization of b2evolution. Most of the time, calling your blog through index.php with a skin will be enough. You should try to customize a skin before trying to use this file."
Note the line:
$skin = '';
which is what effectively disables the use of a skin.