FreePBX is a Standardised Implementation of Asterisk that gives you a GUI to manage your system. If you have looked into Asterisk, you would know that it does not come with any built in programming. You cannot plug a phone into it and make it work without editing configuration files, writing dialplans, and various messing about. FreePBX simplifies this by giving you a pre-written set of dialplans that allow you to have a fully functional PBX pretty much straight away.
This package only contains the sources, and should be installed with smeserver-freepbx