This is a Google "dork" or advanced search operator. It tells the search engine to only show pages where the specific words follow it in the HTML title tag of the webpage.
When combined, you are essentially asking Google: "Show me every web server on the planet that is accidentally or intentionally exposing a folder full of music files." The Experience of an Open Directory
Most files found via this method are copyrighted material. Using these links to download music without paying the artist is a violation of copyright law in most jurisdictions. Intitle Index Of Mp3
Even in the era of $10-a-month unlimited streaming, the "Index Of" search remains a cult favorite for those who enjoy the "thrill of the hunt" in the digital landscape.
In some cases, these are intentional "warez" sites designed to bypass traditional copyright takedown methods by staying under the radar of main search results. The Risks and Ethics This is a Google "dork" or advanced search operator
This is the default title given to a directory listing on a web server (typically Apache or Nginx). When a web administrator forgets to put an index.html file in a folder, the server simply lists every file in that folder for the world to see.
A link to go up one level in the server’s file system. File Name: Usually the artist and song title. Using these links to download music without paying
The "Intitle: Index Of Mp3" Phenomenon: A Guide to Open Directories