Sometimes, for one reason or another, you will have no index file in your directory. This will, of course, mean that if someone types the directory name into their browser, a full listing of all the files in that directory will be shown. This could be a security risk for your site.
To prevent against this (without creating lots of new 'index' files, you can enter a command into your .htaccess file to stop the directory list from being shown:
Options -Indexes
Keywords: How to Stop A Directory Index From Being Shown