In theory, it finds unprotected server folders where someone has uploaded Athadu.mp4 (or files with similar names).
To understand the intent behind the keyword, we must break it down into its three core components:
This is the string found in the title of a default Apache or Nginx web server directory listing. When a web administrator misconfigures a server and does not upload an index.html file, the server displays a plain-text list of all files and subdirectories in that folder. The title of that page is almost always Index of /.
By combining intitle:index.of, you are filtering the entire internet to find only these raw, unformatted directory listing pages.