This program is used in our virtual servers. It works as a replacement for the init program, which is usually the first program to be run after the kernel is loaded. In a virtual server (jail) there is no such thing. This makes it rather difficult to restart a virtual server from the inside, as well as from the outside.
Hence the jailinit program. It is the first program to be started in a jail and responsible for starting up all other programs and services. After finishing this it goes to sleep until it receives a SIGHUP. When this happens it will kill all other processes in the jail and restart it.
If it gets a SIGTERM signal instead, all processes will be halted and the jail will go down cleanly.
You can allways find the PID of the jailinit process associated with jail X by running the command:
cat /path/to/jailX/var/run/jailinit.pid.

You can get the source code here. This latest version works on both FreeBSD 4.x and FreeBSD 5.x !
To compile jailinit properly, you may want to use this Makefile. This will create a statically linked binary, so you can even use it in a jail without any shared libraries, which is a Good Thing (TM) securitywise.