The magic word ROOTPAGENAME is described at Wikipedia:Magic words#Parser functions. An example of its output is {{ROOTPAGENAME:Template:Example/with/subpages}} → "Example".
This template should never be used, but when someone uses the magic word "ROOTPAGENAME" with a piped parameter, it will prevent breakage. Parameter 1 will be used as the : parameter, other parameters will be discarded.
The page will be added to the hidden category Category:Pages which use a template in place of a magic word (3).
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