that's not a subdomain. Subdomains are mail.example.com , secure.example.com , pages.example.com , wiki.example.com and so on
The stuff after domain is just a path to a script or resource (page) that's served by the web browser to you.
For example, the web server may transparently (in the background) convert example.com/RIGHTTHERE to example.com/serve_page.php?keyword=RIGHTTHERE
The web server / script would read the keyword and send you data according to the keyword