DFU stands for Device Firmware Update which reloads the software and the firmware of your device. This reload could be used in case the iPad is set up correctly (more about it you can find in How to set up iPads?) but not working fine and when the first troubleshooting steps for an iPad weren't helpful.
To make the hard reset in DFU mode (Device Firmware Upgrade), please, follow these steps:
1. Connect the device to your Mac computer using a USB cable.
NOTE: Make sure that you're using the latest version of iTunes.
2. Hold down both the Home and Lock button for 10seconds.
3. Then, release the Lock button while continuing to hold down the Home button for another 10 seconds.
NOTE: If the Apple logo appears on the iPad screen, the Lock button was held down for too long.
4. If everything done successfully, iTunes will alert you that the device was detected in DFU recovery mode. The iPad screen in this case will remain switched off.
NOTE: If your device shows a screen telling you to connect the device to iTunes, retry these steps again. If you can’t update or restore your device with recovery mode, or you can't use recovery mode, please contact Apple Support.
The video tutorial regarding how to put the iPad in DFU Mode could be found HERE.