DFU stands for Device Firmware Update. DFU mode is used to bypass the bootloader, which is where many iTunes errors originate from. Often times if you receive an error on iTunes while in the normal state or in Recovery Mode, placing the device into DFU mode will fix the error.
Hello - from the above the issue relates to entering the 'passcode' too many times - here's yet another LINK that is more specific to the situation - good luck! Dave