Back button focus is a focus technique often used by professional photographers who want to independently control their camera’s autofocus and the shutter. By default, cameras usually activate autofocus when you press the shutter button, but there are many instances where it is actually preferable to separate these two controls into two different buttons.
This is a very important technique that can improve both image sharpness and compositions, so long as you understand why and when you would want to use it. I have written a huge in-depth (5000 word!) guide that explains exactly what back button focus is, when you should use it and how to se it up with your camera.