Back-Button Focusing is the Swiss-army knife of the various focusing modes. It is easy to setup, use and a time-saver in a wide range of situations.
By default, focus acquisition and shutter release are coupled to a single button – the shutter release. Half-pressing activates and acquires focus, while a full press releases the shutter for the image to be taken.
In contrast, back-button focusing separates these two functions and reassigns focus acquisition – usually – to the AF-ON button.
The benefit of this is  to enable single point and continuous focus from a single button. Pressing and releasing AF-ON offers single point focus and pressing and holding AF-ON offers continuous focus. When not set up, the focus mode dial must be toggled to switch between these modes – a fiddly and time-consuming process; and  to enable pre-focusing and lock-on to a focus point. Pressing and releasing locks single point focus and makes it possible to take a number of frames at that focus distance without engaging the focus motor – an effective way to capture a series of images without uncertainty about the accuracy of focus.
Set-up (using X-T2 Firmware v4.0)
To set up back-button focus quickly  Use or assign the AF-ON button and  SETUP > BUTTON/DIAL SETTING > SHUTTER AF OFF
Use it when  single point focus and continuous focus are needed interchangeably,  distance to the subject is fixed and there is little or no movement,  in low light photography with a deep depth of field and  for HDR images.