DFU means Device Firmware Update. If a restore using Recovery Mode doesn’t work you will want to use DFU Mode as a last resort. When placing your iPhone into DFU Mode it does not load the OS before attempting the restore
Put the iPhone into DFU mode so you can do an iTunes firmware restore follow these steps:
Open iTunes and connect the iPhone to your Mac/PC.
Press and hold the Home button and the Sleep/Wake button at the same time.
After exactly 10 seconds release the Sleep/Wake button. Continue holding the home button until you iTunes pops up a message telling you that it has detected an iPhone in recovery mode.
The iPhone screen will remain black like this: