The Raspberry Pi serial console is a method of accessing your Raspberry Pi. The system spawns a Linux terminal which is accessed over two pins on the Raspberry Pi's GPIO. While also being a normal terminal there can also be some debug information displayed via this console which is handy if your monitor or network isn't working.
We're planning on shipping a USB A to USB B 2.0 cable with a 1-1.8 Metre length with the clip. The cost extra covers the cost of the cable and extra postage in the higher volumes. You may already likely have these cables sitting around as they are the same type included with most printers and some Arduinos.