It appears to be a very simple circuit. The purpose of the diode is to prevent VCC being provided by another source from being fed into an Arduino Nano's 5V pin when the Nano is being powered by the USB port, and when a jumper is mistakenly left in-place. I believe on the Arduino Nano, the 5V pin is an output pin, which puts out 5V. So it wouldn't be a good idea to have VCC coming from another source colliding with the 5V pin. Therefore, a diode is put into the circuit to prevent something like this from happening.
The schematic diagram shows a 1N4848 being used here. I read the data sheet, and it seems to be quite beefy for the actual application. I tried looking for a 1N4848, but I can't find any at the local electronics store. I have a ton of 1N4148. Since both the VCC and 5V falls within the 1N4148, I feel that I can use this in-place of the 1N4848. Other things in the data sheet such as maximum forward current, maximum reverse voltage, although all important, I don't think they are in play for this particular application. And, the power supply I'm using is well regulated at 5VDC.
Thanks!