Sorry you got downvoted. It's a legitimate question. I think the parent comment was being ironic by pointing out a dangerous command that might be run. I think it would remove any files at the root directory.

rm -> remove/delete the given file

-r -> recursively delete files and subdirectories

-f -> remove forcefully without command prompt, specifically useful when deleting unwritable files

/ -> the root partition, basically the base of the entire system

When combined, it's dangerous because it could delete every file and directory on the root. I'm not sure, but I think you would need superuser (sudo) privileges to do so, but I'm not gonna test it:

sudo rm -rf /

Here's more on rm: https://www.geeksforgeeks.org/rm-rf-command-in-linux-with-ex...