This image has little artistic merit, but it does show how focus points work. I just shot this in my back garden using our resident Robin just for you.
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We have branches both in front and behind the Robin, the camera desn't know which you want to be in focus so you have to tell it.
By selecting the centre focus point I can centre the part of the image I want to be in focus, the Robins eye, I half press the shutter which holds that focus point then I can frame the picture as I want keeping the shutter release half pressed, hoping the Robin doesn't move. When I'm happy I press the shutter and take the picture.
Left to it's own devices the camera could just as well focus on a branch in front or behind the bird, this way I tell the camera where I want it to focus which is important in this shot.
This is more impoprtant if you get close to the subject and or use a larger aperture (smaller f number) either of which reduces the depth of field which is that part of the image from front to back that is in acceptable focus.
If the lighting had been more dificult I could also make the camera use the same area for setting the exposure by linking the AF and the Ae in menu so the part that is in focus is also the middle tone of the exposure ensuring that the bird (in this case) is both in focus and exposed correctly.
Hope this helps Chris