The job of the busboard is to distribute the voltages generated by the powersupply to the individual modules.

It takes three or four cables from the powersupply (+12V, -12V and Ground and sometimes +5V) as input and has 2-rowed 16 pin headers (typically keyed to avoid damage by plugging stuff in the wrong way) which allow for a connection of multiple modules. Usually DIY doesn't really pay off with busboards because the box headers are too expensive.

Some busboards have a IC on board to generate the +5V from +12V.