1) You got an operating systemin stalled on a partition
2) You mount that partition
3) You mount a couple of directories needed to do certain work (dev, sys and proc)
4) You run "chroot /media/disk" and you are "in" the system you chrooted too. You can update, upgrade, install grub, edit files - well, you can edit by just mounting, duh - you can rebuilt intiramfs, etc
5) You exit the chroot
As chroot can be done from liveCD's, you can repair seriously broken systems (sometimes).
You only need to copy and paste the commands, but in general its three
mount the partition
mount the direcotires needed, i admit: with weird looking options
chroot
(wait a second: that are only two commands: mount and chroot ...

This way, that way, good luck.