Jack Tools is an OS X implementation of Jack, and it installs everything necessary to take full advantage of Jack on OS X.
Jack (the Jack Audio Connection Kit) is a low-latency audio server, written originally for the GNU/Linux operating system, and now with Mac OS X support. It can connect any number of different applications to a single hardware audio device; it also allows applications to send and receive audio to and from each other.
What's new in Jack Tools
Version 0.51 only fixes bugs. It adds no additional functionality.