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Autofocus is usually done by pressing the shutter button halfway down: I focus, I keep the shutter button down to compose my scene, then I press it all the way down to take the picture.

The “back button focus” technique consists in assigning the focus function to another button on the camera (usually at the back, controlled by the right thumb, hence the “back button”). This way I can focus and compose freely, without having to hold down the shutter button.