If you visit yourdomain.com through a browser basically it will take you to "yourdomain.com/index.html" (or index.htm,index.php....). The same way if you browse "yourdomain.com/subdirectory" then it will show you "yourdomain.com/subdirectory/index.html". That means if you wanna browse any directory(of any website) through a browser the directory must have a index.html or similar file to show your desired page. Again if the directory don't have any index.html file and have other documents(.mp3,.doc......) you'll get a list of all documents and also subdirectories of that directory. This option can be disabled too.