Questions in running rsync example

[ying lcs]

Hi,

I am trying to run the 'backup to a spare disk' in this example:

http://samba.anu.edu.au/rsync/examples.html

My question is
Why i need to to this:
mount /backup/$d

If i have an external disk, it will be under /media/disk directory,
why I need to mount?

And what is the meaning of  '-exclude fstab' here?

rsync -ax --exclude fstab --delete /$d/ /backup/$d/

Thank you.