DFU stands for Device Firmware Update. If restoring your device using recovery mode does not work then this is the last method that you’ll have to use.
Here is the step by step guide on how to put your iOS device into DFU Mode.
Open iTunes and connect the device to your PC/Mac.
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 iTunes pops up a message telling you that it has detected an iPhone in recovery mode.
Restore your device to the latest firmware.