sudo rm -R / - never!
If someone tricks you into typing this, you’ll erase everything off of all of your mounted hard drives. Not just the system drive, but all of them!
rm means remove. -R means recursively, that is, every directory inside the target. / is the target, and means the top level of your computer, which logically represents all of your mounted hard drives.
So, don’t do this. That would just be silly.