I’m pretty sure I just made a typo there. Sorry about that! The “-r” is for “recursive”, I believe, meaning “delete this directory and all it contains.” Flags like -a, -r, -f, etc. whenever they are used can change based on the command being given. If you’re ever in doubt, check the “man page” for the command you want to know more about. On the command line just type "man ", i.e. “man ls” or “man rm”, and it will give you answers to most of your questions.
I can’t check the exact phrasing of -r for rm right now because I’m not on my Linux machine, but I do seem to remember it being for “recursive” with that particular command.