What is chroot?
"chroot" is a way to change the / folder to another one.
For example: I have a system inside /dev/sda1, but the grub bootloader is broken, so I boot into a Linux live cd and chroot into it to acess my operating system installed at /dev/sda1.
How to chroot into my system and recover grub?
Simple. First you need to mount /dev/sda1 to /mnt:
$ sudo mount /dev/sda1 /mnt
Now, mount your /dev, /proc and /sys to the chroot folder.
$ sudo mount -t proc /proc /mnt/proc
$ sudo mount -o bind /sys /mnt/sys
$ sudo mount -o bind /dev /mnt/dev
Now, you are good to go.
$ sudo chroot /mnt
And now, you can simple do update-grub
And now, just type sudo reboot.