In this case, I think the Linux man page is rather lacking.
One tip---many Linux man pages are really horrible. Often going to www.freebsd.org
will enable you to find better man pages.
Hrrm, looking at the BSD man pages for root, though they're quite clear if you're experienced, I could see them being confusing too.
When you log in, your root directory, that is, the base directory of the system, is / (root.)
Doing chroot makes it a different directory. This is sometimes used, for example, if you have Fedora on one partition and Ubuntu on the other, but need to run some commands from inside the Ubuntu system.
So, first you mount the Ubuntu partition, say on /mnt. Then you also mount, with the bind option, /proc and /dev.
Now, you can do
chroot /mnt /bin/bash
You will now be working as if you had booted up into the Ubuntu system.