PAGENAME is an expression (one of the "Magic Words") used on thousands of our pages in a template (often hidden within another template). It reproduces the name of the page it appears or is transcluded in, after cutting off any namespace prefix such as "Talk:", "User:", "Image:".
One very common use is to place a page near the top of the listing for a category of the same name, using just "[[Category:{{PAGENAME}}| ]]".
If you want the namespace prefix included, use "FULLPAGENAME".
On a subpage, use "BASEPAGENAME" to cut off the final slash and what follows it.