DFU means Device Firmware Update. This is a state that you can put your iPhone in an interface with iTunes but does not load the iPhone operating system or boot loader. You can change and choose the firmware you want to install on your iPhone in DFU mode. Otherwise, if there are some errors when degrading your iPhone 6s, you must put your device in DFU mode to restore or degrade the firmware.
As is common in iPhones, this is the way to login to DFU mode, but in the seventh generation, Apple changed this method to the following method.

Method for your iPhone to login DFU mode

1.Be sure to install the latest version of iTunes on your PC or Mac.
2.Connect your iPhone, iPad or iPod touch to your computer or Mac.
3.Make sure iTunes is running.
4.Turn off your iPhone, iPod Touch and iPad if it is not already on.
5.Press and hold the On / Off button on the right side (for iPhone 6 and above) or on the top (for all iPhone, iPad and iPod devices) for 3 seconds.
6.At the same time, pressing the volume down button with the On / Off button for 10 seconds (if you see the Apple logo, you have pressed for a very long time, and you have to restart the process.)

How to Login to DFU mode in iPhone 7 and 7 Plus

How to Exit DFU mode with Your iOS device

If your iPhone is stuck in DFU mode, the easiest way to get it out is to simultaneously press the power button and home button for about 10 seconds until the Apple logo appears. This way of working for all iPhone, iPad, and iPod touch 5, iPod touch 4.