index.html is the document page that is present in the directory from which pages are supposed to be served.
Like example.com/mypage.html will open mypage.html
and example.com/index.html will open index.html
and just example.com will /also/ open index.html (OR any other file name that has been specified to the server and has higher priority than this. like index.php, index.py, home.php etc.)
hm, it doesn't have to be "index.xxxx", asp.net built websites define it as Default.aspx, it's something that you specify in your server's settings. index.html, etc are the default one for Apache web server.