This outputs the link tags inside the header.
The purpose of link tags is to specify links that related to the current page. The rel attribute specifies how these links are related.
<link rel="stylesheet" href="whatever.css" type="text/css" media="screen" />
This specifies where the stylesheet for the page is, when you're displaying it on the screen. If the media said "print", then it would be a stylesheet that would be used when the page was printed.
<link rel="alternate" type="application/rss+xml" title="RSS 2.0" href="whatever" />
This gives an alternate version of the page. In this case, it gives the RSS2 feed for the page.
<link rel="pingback" href="http://example.com/blog/xmlrpc.php" />
This gives the pingback address, for automatic pingback functionality.
<link rel='archives' title='May 2007' href='whatever' />
And finally, this one tells where archived versions of this page can be found.
Some link tags are used by the browser, some are used by search engines or other tools. But *all* link tags are not meant to be directly seen by the end user, they're a form of metadata about the webpage itself, specifying other related webpages. They are used exclusively by browsers, search engines, and other automatic tools.