A portrait’s composition is the sum of how you place all the parts within it: use of the edges of the frame, use of shapes within the frame, the prominence of any foreground/background details, the position of the subject within the frame, even the shape of the frame itself.
There are hundreds of guidelines and ‘rules’ for composition in photography. In general, these are intended to create attractive compositions. Depending on your desired intentions, you may want to achieve something else entirely. In addition, relentless applying the Rule of Thirds or The Golden Ratio to every shoot will inhibit your creativity and result in images that look repetitive and uninspiring. Developing your own compositional style will help set you apart from other photographers, so learn these rules but also experiment with (intentionally) breaking them.