Welcome to WebmasterWorld Guest from 188.8.131.52
Lets imagine your site is about cars, you have a folder called "engines". I might get to that folder by following a link to a page about ford engines...
This might be of interest so I then decide to delete the ford.html so see what other information you have and get a page not found. The index page could have been used to allow me to see all the pages available. By default some servers, if not most! will show a directory index. "Index of" page. These are very unprofessional, you can make a simple html page to link to your pages within the folder.
I have, in fact, come across site folders without an index file and it will show an index of all the files there.
Also part of the server configuration - what you should get here is a server error message, "Directory Indexing Not Allowed" (paraph.) If you can't get this changed, putting the index file in that directory is a fix, but it may be indicative of other small oversights that add up to security issues.
RewriteRule ^scripts/?$ /this.file.does.not.exist [L]
RewriteRule ^styles/?$ /this.file.does.not.exist [L]
RewriteRule ^images/?$ /this.file.does.not.exist [L]
which can be simplified to:
RewriteRule ^(scripts¦styles¦images)/?$ /this.file.does.not.exist [L]
There's already a redirect in place which strips any named index file filename off the URL request.
With this, they don't get a "there's a folder here and you are not allowed to look at it" message, they instead get a "that thing doesn't exist" error message.