Directory listing is a webserver feature where the webserver will automatically generate an HTML page that lists the files in a directory when there is no index.html or index.php file.
Directory listing can be enabled through .htaccess files.
For example, to enable directory listing for the path /download, create the file:
download/.htaccess
with the contents:
Options +Indexes
You can control how the directory listing appears by adding an IndexOptions directive. For example, the following will display the listing without file type icons and will also display file sizes and modification times.
Options +Indexes IndexOptions FancyIndexing SuppressIcon
See the Apache IndexOptions documentation for more information on customizing directory listings.