Debug bus !!
Hmm....name itself says
it is used for debugging purpose.
This bus will have the critical signals of ur code which u want to check if there is an problem occurs
A simple example... u wrote an state machine and its got hnaged some where, In real time scenario, u donno which state it is in. U connect the state status to the debug bus ... U can check the status of the state by looking in to the debug bus.
in a single line, Debug bus is used to check the status of the critical signals of the chip