These commands tell search engines to look specifically for the text generated by server software like Apache, Nginx, or IIS. The Risks: Security and Safety
If you are a site owner and see your files appearing in "Index of" searches, you should take immediate action:
Files in open directories aren't always vetted. A file named setup.exe could easily be a virus.
In your .htaccess file (for Apache), add the line: Options -Indexes .