If a directory is requested, and the there is no DirectoryIndex (e.g., index.html) in that directory, then the server by default will return a formatted listing of the contents of that directory.

To turn these directory listings off, simply add to (or create) an .htaccess file in the directory you want to protect and insert:
Options -Indexes

(Or, the easier solution is to upload an 'index.html' file into that folder.)