Besides the 54 digital I/O pins and 16 analog inputs, it also has 4 UARTs (hardware serial ports), a 16 MHz crystal oscillator, a USB connection, a power jack, an ICSP header, and a reset button.
It provides everything needed to support the micro-controller.
You will simply connect it to a computer with a USB cable or power it with an AC-to-DC adapter or battery to get started.