Focus is plain physics. What you (or your camera's auto focus) actually focus on should be sharp...and so should anything that is exactly the same distance away from the camera. When you use a large aperture (small F number) your DOF (Depth of Field) might be very thin...so things that are closer or farther than your focus point might be out of focus.
So for example, if you are shooting a person and using a large aperture (thin DOF), you will almost always want the eyes to be in focus. So if you focused on their nose, or something else, their eyes might not be sharp.
So if you want to take your photo, without the eyes being directly in the centre where your AF point is...most cameras will allow you to use another AF point, so that you can focus on something that is not in the centre.
Or you can focus on the eyes, lock the focus, then recompose the shot to fit the composition you like.