You usually focus a digital single-lens reflex camera (DSLR) by pressing halfway down on the shutter button, and then take the picture when the button is pressed in fully. Back-button autofocus makes it so the shutter button doesn’t control the focus activation at all, but instead makes another button on the back of the camera (hence the name) activate the focus feature.
You can use back-button focusing for many things, but the most common is when taking portraits or when you have a still subject for an amount of time. Say what you want is to focus on a subject and take multiple shots without having to refocus. That is the reason back focusing exists. You can use this feature until the subject moves, then you will have to recompose the shot and achieve focus again.
Go into your control settings and change the shutter control to Meter Only and your AF-ON button to Metering and AF Start.
Achieving constant focus on a moving subject can be hard to do. Use back-button focusing as a way to follow your subject and keep them in crisp focus. To do this turn your Focus mode to AI Servo and choose the focus point that will be able to follow your subject. Move the camera and hold down the BB Focus button as you hoot so the subject is always being followed by you autofocus.
Photo by Tyler Judson 
Photo by Tyler Judson
Back-button focusing also allows you to switch from manual and autofocus easier than flipping the switch on your lens. Whenever you need manual focus, s
Back-button autofocus can be hard to understand and adapt to, but you will be able to master it with some hard work and practice!
