Short answer: Nothing

Long answer: It is part of a command that deletes everything. The only thing missing is the argument specifying what to delete. Examples:

rm -rf *
rm rf /some/directory

It’s somewhat (in)famous because it’ll do so without asking for confirmation. The only exception is rm -rf / on a modern distro which will complain that you’re attempting to delete EVERYTHING on the system. In the olden days it’d just do it, but these days it tells you to add –no-preserve-root as well if you really wish to do so.