The rule of thirds is a concept in image, video and film production in which the frame is divided into into nine imaginary sections
In most "people shots", the main line of interest is the line going through the eyes. In this shot, the eyes are placed approximately 1/3 of the way down the frame
Using the Rule of Thirds helps produce nicely balanced easy on the eye pictures. Also, as you have to position things relative to the edges of the frame (mentally dividing an image into nine equal parts, then dividing the image into two equally-spaced horizontal lines and two equally-spaced vertical lines) By placing an image at these points it helps get rid of ' tiny subject surrounded by vast empty space' syndrome. Therefore it creates an interesting, balanced shot which is focused.
In photography, headroom is a concept of composition that addresses the relative vertical position of the subject within the frame of the image.
Headroom refers specifically to the distance between the top of the subject's head and the top of the frame.
If the head room is too little or too much it may make the focus of the subject will not be clear and uncomfortable on the viewer where as if the headroom is sufficient (preferably 1/3 down the frame - following the rule of thirds). This should make the portrait or positioning correct.