This command is part of the posix extensions and needs to be imported to be available to the shell.
See Posix Commands
|-r||recursively delete files|
|-rf||both -r and -f|
Removes each argument. If -r or -rf is specified and the argument is a directory, then recursively deletes all files in the directory first.
If -r is not specified then directories are not deleted (see rmdir) to remove directories.