The Web server allows you to view a list of files in a directory in case there is no default file (index.php, index.html or index.htm). By default it is disabled and you will receive the error 403 Forbidden.

If you want to enable it for any folder, create a file named .htaccess (including the dot at the beginning) and insert this rule in file:

Options +Indexes

This option is valid for the given directory and all its subdirectories.