This is the {{skin-file disambig}} template.

It holds the fall-back message for the magic links Special:MyPage/skin.css and Special:MyPage/skin.js. The code that makes those links work was added to MediaWiki:Common.js in October 2009. Those links only work when all the following things are true: User is logged in, user has javascript enabled in his browser, the code in MediaWiki:Common.js is still there.

This fall-back message is loaded by system messages at the server end, so it "always" work. (It needs to be added to some more system messages so "always" isn't entirely true at the moment.)

When this template is placed in system messages it should be surrounded by code that only makes it load on user /skin.css and /skin.js pages. Like this:

{{#switch: {{lc: {{SUBJECTSPACE}}#{{SUBPAGENAME}} }}
| user#skin.js  
| user#skin.css = {{skin-file disambig}}
}}

This template could have contained that code itself, but that would be less efficient since then this template would have to be loaded every time those system messages are used.

The above code also triggers in user talk space, upper-case /SKIN.CSS and on sub-subpages like User:Example/test/skin.js. That's on purpose, since it is more helpful to catch more cases.