"The FreeBSD jail mechanism is an implementation of operating system-level virtualization that allows administrators to partition a FreeBSD-based computer system into several independent mini-systems called jails." source: Wikipedia
A jail is characterized by four elements:
A directory subtree -- the starting point from which a jail is entered. Once inside the jail, a process is not permitted to escape outside of this subtree. Traditional security issues which plagued the original chroot(2) design will not affect FreeBSD jails.
A hostname -- the hostname which will be used within the jail. Jails are mainly used for hosting network services, therefore having a descriptive hostname for each jail can really help the system administrator.
An IP address -- this will be assigned to the jail and cannot be changed in any way during the jail's life span. The IP address of a jail is usually an alias address for an existing network interface, but this is not strictly necessary.
A command -- the path name of an executable to run inside the jail. This is relative to the root directory of the jail environment, and may vary a lot, depending on the type of the specific jail environment.
source: https://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/jails.html#jails-synopsis
