Serial supports the BeagleBone Black (BBB) embedded Linux development board in one of two ways:
The easiest way to connect the board to your Mac is directly, using a USB cable. The BBB will power itself directly off of the USB port, and after the board has fully booted into Linux a console port will appear within Serial called "BeagleBoneBlack". The baud rate and other settings are ignored when connected via this method. The drawback to this method is you won't be able to view any console output created during the boot process.
The second way to connect to the board is using the serial port header on the board. You'll want to connect the board this way if you need to diagnose issues with the boot process, or while flashing the on-board MMC device. Use a compatible serial cable, and configure Serial for 115200-8-N-1. For suggested cables and pinouts, refer to the BeagleBone Black documentation.