By default, if you attempt to request a directory listing (e.g. http://domain.com/files/), Apache will attempt to read an index page (e.g. index.html or index.php). If there is none, it will reject the request with a “403 Forbidden” error message.
To enable directory listing (indexing) for a given directory, just put the following:
Options +Indexes
into a .htaccess file in document root directory (or a subdirectory thereof). This will make Apache display a list of all files in the directory instead.