This tile is included in baseLayout.jsp within web/WEB-INF/tiles-config.xml. Matt Raible included a comment that indicates that the way header is used here demonstrates the use of a Tiles Controller.
Within the <definition> of baseLayout, Matt has the following line:
<put name="header" value=".header.userCount">
Thus, header is refering to yet another tiles definition.
Down below, at line 72, we see where .header.userCount is resolved:
<!– Header definition – shows user Count using a Controller –>
<definition name=".header.userCount" path="/common/header.jsp"
Thus, header.jsp displays the number of active users in the top right portion of the page.
The next block of logic in header.jsp checks to see if the logged in user's role is tomcat or admin. If so, the link to the Main Menu is inserted at the top of the page, so it will be available for any page based on baseLayout.jsp.