I keep reading about back button focus, what is it and when is it useful?

Melissa Matthews, Preston

Brian Says… Instead of focus being activated when the shutter is halfpressed, it is activated by pressing one of the back buttons, usually AF-ON or ‘*’. It is popular for action photography, but once you train your brain to use it, it can be used all the time. You can press the back button to focus, then release and recompose – even in AI Servo. Alternatively, keeping the back button pressed when AI Servo is active makes the camera track the subject.

Back-button focus is popular for sports and wildlife, but is useful in many other scenarios