Modern SLRs are highly-complex technically advanced devices. One of the complex technical bits that we all use but don’t really understand is the auto-focus system. Your typical SLR has an array of 5, 11 or even more points arranged in a grid pattern that it evaluates for focus. Left to itself, in auto-focus mode, it will decide which of the points is aligned with the subject of the photo and will use this point to focus the lens. In effect, you are letting the camera chose the subject, which may not give you your intended result.
You can instead chose to select the focus point that is to be used in the grid. While this is theoretically a better approach, picking out the right one of 11 or 19 or more points while trying to deal with all the other elements that have to be controlled is a daunting task, one that I have never mastered.
I prefer to turn off all but the centre focus point. Now, we all know that it is frequently the case that the subject of the photo should be almost anywhere except at the centre, so what you do is line the centre point of the camera up on the subject – typically on an eye if dealing with a person or animal – and press the shutter release button half-way down. The camera will focus the lens – making a beep sound if you have forgotten to turn off that feature – and will not change the focus as long as you keep the button half-pressed. So now you recompose, moving the camera to get the subject nicely on one of those magic thirds, without releasing the button or changing position, and then finish pressing the button to take the picture.