It’s a shield that allows implementing a full-speed USB peripheral/host controller through the necessary digital logic and analog circuitry it contains. What does this mean? Basically, your Arduino will be able to actually interface and control any USB slave device such as digital cameras, thumb drives, Bluetooth dongles, you name it. Also, this new version corrects the pin out for the GPX and RESET pins.
This shield connects the Arduino's hardware SPI pins (D10-13) to the MAX3421E since a four-wire serial interface is what communicates with the host controller chip. A USB having the type A female connector is what gets wired with the IC, and just like any USB, this one provides a 5V as well. This product basically takes the power it needs from the Vin pin which is located on your Arduino, this power is then regulated to 5V and 3.3V, and last but not least, the SPI signals gets lowered to 3.3V through the hex convertor.
Product pictures and description are of Sparkfun.com