Chrooting can be super useful for changing things like the root password, repairing grub bootloader etc., things that require the system to be mounted. Typically if your chrooting into an OS drive you can boot up on a Live Linux distro, or use the Rescue feature on some Linux installers.
Mount main drive
Change sdb2 to the root partition of your drive.
mount /dev/sdb2 /mnt
Mount other stuff
mount -t proc none /mnt/proc mount -o bind /dev /mnt/dev mount -o bind /sys /mnt/sys
If you want to, you can mount the boot partition. Change sdb1 if your boot partition is something else.
mount /dev/sdb1 /mnt/boot
You should now be inside the chroot environment. To exit the chroot, hit control+d or type exit.