DFU stands for Device Firmware Update, and it is the deepest type of restore you can do on an iPhone. It is a mode where the BootROM can accept iBSS, and it allows all devices to be restored from any state. DFU mode is used to reload the software and firmware of an iPhone, and it is the last step in determining whether an issue is caused by software or hardware.

Here are the steps to enter DFU mode on different iPhone models:

• For iPhone 6s and below, iPhone SE and iPod touch: hold the Home button and the Lock button until the device reboots.
• For iPhone 7 and iPhone 7 Plus: connect the device to a computer using a USB cable, hold down both the Side button and Volume Down button, and after 8 seconds, release the Side button while continuing to hold down the Volume Down button.
• For iPhone 8, iPhone 8 Plus, iPhone SE (2020) and iPhone X or newer: quick-press the Volume Up button, then quick-press the Volume Down button, then hold down the Side button until the device reboots.

When an iPhone is in DFU mode, nothing will be displayed on the screen. If open, iTunes will alert you that a device was detected in recovery mode. To exit DFU mode, simply force restart your device.