There is not much to understand.
You have the Atmel microcontroller,all of its details are available at www.atmel.com
You have a USB interface adapter to connect USB to the serial port.
You have a couple of voltage regulators.
You have a voltage comparator that selects the USB source for power if the external power is too low.
Couple of indicator lights. Connectors.
That's about it. Read the datasheet to understand what the microcontroller can do: memory, A/D, PWM output, digital I/O, interrupts.