The phrase "Index of" is a standard header generated by Apache and other web servers when a folder lacks a default homepage (like index.html ), exposing a raw list of files [3]. The term is the URL-encoded character for an exclamation point ( ), and the word "!!BETTER!!"
The phrase in your query suggests you may have encountered this term in the context of "repacked" or pirated software/media listings, where uploaders add such tags to claim their version is superior. What is an "Index of Mp4"? Index Of Mp4 %21%21BETTER%21%21
If you are a site owner, seeing your files in these results means your server is "leaking" information. You should disable directory browsing in your .htaccess file to keep your data private. The Bottom Line The phrase "Index of" is a standard header