The nasal septum is the wall that divides the inside of the nose into a left airway and a right airway.  When the septum doesn't sit in the middle, it can obstruct airflow through the nose instead of guiding the airflow.  We then call it a "deviated septum," and that deviated or crooked septum can be straightened during a rhinoplasty.

Most commonly, the deviation of the septum cannot be seen from the outside of the nose.  The deviation that obstructs the airflow is deeper inside the nose, where you can't see it.  This patient's septum was so bad that you could see it before surgery.