This is where you plug in the USB cable for programming. It is also one of the two ways of providing power to the board.
One end of the USB cable will attach here while the other end will got to a USB port on the computer you use to write your programs. We'll talk about how to actually do all that later.
If you are using the USB cable from the Go Bundle, then you're set. If you are using one of your own USB cables, make sure it is a good one. While the world is full of USB cables, not all of them are made equal.