Chroot allows execution of a progam within a "root directory" other than normal system root. It effectively forces all programs called to use the chrooted directory as the CWD. This has the added security of forcing applications to run within so-called chroot-jails.
This can be useful when preparing and testing software for source or binary distribution, and when recovering crashed systems. You can find out the various options for chroot from this online man page.
A useful set of utilities to create a chroot for security reasons is Jailkit.