The septum is the mid-portion of the inside of your nose which separates the two sides. It is made up of cartilage in the front part and bone in the back part of the nose. Normally it is straight and aids in making sure air flows in a laminar fashion.
In some cases, however, it can have a spur created by an extra piece of bone that grows out from the septum, and this has two main effects;
It causes turbulent or no airflow and this will result in a permanently blocked nostril.
The other effect is that the nose can look skew due to the septum being the main support structure to the nose.
A septoplasty is an operation to repair a deviated septum or one that has a spur. Depending on the severity of the problem the surgery can be done in an open fashion (Septo-Rhinoplasty) where an incision is made through the skin between the nostrils (Columella) or in a closed fashion whereby the cuts are all on the inside of the nose.