frogs is used as a ``metasyntactic variable'' -- a
conventional placeholder for some file, directory, text, graphic, or
executable program that a real user would actually refer to. (Similarly,
``George Spelvin'' is a conventional placeholder for a typical user, and
``bourbaki'' for a typical Unix workstation.)
In this case, frogs.html is an arbitrary HTML file, viewable
as a World Wide Web page. It's not really about frogs.