The BitBoard is a breadboard add-on module that allows users to build custom circuits and user interfaces for the BitRanger. It is solidly attached to the bitRanger and provides performance friendly stability. It either comes just on its own, or with with 2 pots, 2 buttons and 4 switches which can help you to start designing your own bitRanger interface.
The bitBoard is closely linked to a project called OMSYNTH which is a breadboard based open modular synthesizer by Casper electronics. Several tutorials on how to make circuits are available on the OMSYNTH webpage.