From MEPIS Documentation Wiki
A chroot on Unix operating systems is an operation that changes the apparent disk root directory for the current running process and its children. A program that is re-rooted to another directory cannot access or name files outside that directory, called a chroot jail.
chroot allows you to pass commands to another root (for example you can pass commands to your hard disk root after booting with a Live CD).
Syntax is as follows:
chroot <path of the target root> command
chroot /mnt/hda1 apt-get install network-manager
This command will install network-manager on the hard drive. hda1 has to be mounted for this command to work.
One possible use of chroot is to repair an unbootable system