|This is a metatemplate; that is, a template that will rarely be used directly in articles, but rather used in other templates which will be used directly.|
This is the "Main other" template, borrowed from Wikipedia.
This template usually takes two parameters, like this:
The first parameter is used if the final rendered page is in the
Main: namespace; the second is used if it's in all other templates.
This is used for various purposes, like in templates which automatically put the pages where they are used into categories, so that they do not put themselves or their documentation into those categories.