The USB plug is how you will need to program and debug your code. We use it to both power your project and also provide the USB serial interface. On older Metro Mini's we used a FT231X chip. On newer Metro Mini's we now have the CP2104 chip. Both act the same but the CP2104 is a little less expensive so we were able to lower the price. For either one, you'll need to install drivers if you are using Mac or Windows. Linux comes with both drivers already.
Windows users can try plugging the board in, Windows update will probably install drivers for you. If the device is not recognized, simply download and install the Adafruit Windows Driver package. When you run it, make sure to click that you want to install the SiLabs CP210x and FTDI chip drivers too!
Mac users can grab & install the FTDI VCP drivers from FTDI If you are running Mac OS X , please use the SiLabs v4 drivers (there's a recently-uncovered bug that makes v5 not work and its fine to use the older drivers).
Once drivers are installed and you've rebooted, you will have a Serial COM port.
The Adafruit Metro Mini is an 'Arduino Compatible' - that means that when using the Arduino IDE or other Arduino-friendly development environments, you can simply treat the 'Mini like an Arduino UNO.
When programming, simply select Arduino UNO in the dropdown menu!