DFU is Device Firmware Update. You should put your iPhone into DFU mode If a restore using Recovery Mode doesn’t work. When placing your iPhone into DFU Mode it does not load the OS before attempting the restore
Step 1: Connect your iPhone to your Mac/PC and press power button for few seconds. You will be asked to slide to power off. Slick it to power off your iphone.
Step 2: Now Press and hold Home button and the Sleep/Wake button at the same time.
Step 3: After 10 seconds release the Sleep/Wake button but keep holding the home button until iTunes pops up with a message telling you that it has detected an iPhone in recovery mode.
Step 4: Once you are in DFU mode you wount see anything on you scree. It will be BLACK.