45 the rm command removes a file/folder
if you want it to remove an non-empty folder it gives an warning to reduce chance you accidentally delete the wrong thing
the -r part tells the rm command to also remove all the files in the folder so you still have a good way of deleting non-empty folders from the command line
the f part tells it to not write anything in the console
the / part tells it what to remove. In this case the root of your system.
If you are a windows user it still might not look that bad as it will wipe out 1 drive or so, on linux however when you install it you give each partition a role (store the users folder (/home in linux), store program data, etc,etc) because of this linux has no real need to separate the drives like in windows and for a user (and this command) there is no way to see how many drives a computer by just using it. (the home folder will always be at /home regardless if the computer has 1, 2 or 20 drives) the command sees the system the same way