What is the mu-plugins folder? – WPMU Tutorials

Users of WordPressMU have long been aware of a folder within wp-content called mu-plugins. Obvious conclusions were drawn because of the name and where it was, but this actually stands for “must-use” plugins. Why? Because any code (plugin) placed in the folder is run automagically, as if it were part of WordPress itself. (Yeah, that label was news to me too.) Support for this folder has actually been in single WordPress since 2.8.
In WPMU, you never saw any reference to this folder from the backend at all. Just drop the code in, and the plugin was already running.
In 3.0, support for this is naturally still there, but the folder isn’t created for you. If you do encounter a plugin (like domain mapping) that needs to run in this folder, simply create it via ftp or through your webhost’s control panel.
Also in the backend, if you are using the mu-plugins folder, is this:
Plugins ‹ WPMU Tutorials — WordPress_1277329518009
A listing! You’ll note on this example site (other than we really need to clean out the plugins we were testing), must-use plugins are now listed. This will only be visible to Super Admins – regular administrators, even
Source: https://managewp.org/articles/13547/what-is-the-mu-plugins-folder-wpmu-tutorials

source https://williechiu40.wordpress.com/2016/10/05/what-is-the-mu-plugins-folder-wpmu-tutorials/