Why mount a folder to a folder in fstab?

I'm on an Ubuntu server with fstab like this:

/dev/xvds /home/ubuntu/stuff auto  defaults,nofail,nobootwait 0 2
/home/ubuntu/stuff /home/bob/things auto  defaults,nofail,nobootwait,bind 0 2

I thought that fstab was for mounting devices to folders. What is the second entry? It seems to mount a folder to a folder, more like a symlink.