{{FULLPAGENAME}} is not actually a template, but instead is a magic word, meaning it is a feature of the Wikimedia software. When used, it will return the name of the page it's on, including the namespace. As an example, when placed on this page, it returns మూస:FULLPAGENAME/doc. When placed on Wikipedia:Village pump, it will return వికీపీడియా:Village pump. It is possible to specify which pagename will appear by using a colon (not a pipe as is typical for templates). As an example, {{FULLPAGENAME:Wikipedia:Village pump}} returns వికీపీడియా:Village pump.

There is actual template code here. The purpose of the template code is to ensure that if someone uses this magic word with a piped parameter, the template code will be invoked and it will not cause an error on the page. Parameter 1 will be used as the : parameter and other parameters will be discarded. This way, if someone mistakenly enters {{FULLPAGENAME|Wikipedia:Village pump}}, the template will change it to the correct {{FULLPAGENAME:Wikipedia:Village pump}}

When a user mistakenly includes a pipe in a {{FULLPAGENAME}} call, the template code will add the page to the hidden category Category:Pages which use a template in place of a magic word. Pages in this category should be fixed by willing editors.