There are several septum's within the body. The term septum simply just means, a divide between two things. If you are referring to the cardiac septum; which separate the heart into left and right sides. There is a top portion of the septum which is called the atrial septum and the bottom portion of the septum; which is called the ventricular septum. They typically keep the deoxygenated blood and the oxygenated blood separated from each other.
If you're referring to the nasal septum, the nasal septum acts as a pathway stabilizer to make sure that the air we breathe in is not "lost" and gets resistance going into our lungs.