The septum is the juncture of the tunicas that surround the CCs. Each CC is "wrapped" in a sheath of tunica, then both are surrounded by another sheath.
Imagine to hotdogs laying side by side. You are viewing them from the end. Each outer wrapping of the hotdog is the tunica. Now imagine a wrap that binds the two hotdogs together. This is the outer tunica. The triangle area where they band together is called the septum.
Lately I've been doing a move that if I had to call it something it would be a candy cane pull. While standing, pull the excess shaft skin back towards your body with one hand. Then using an overhand (palm down) grip, grasp your penis firmly just behind the head, really grip hard. You don't want the tunica to slide under your grip. Now pull straight out, then up towards your chin, while keeping your palm down. This will put your shaft in a candy cane sort of position. Pull with all your might for as long as you can hold the grip without slipping. Change hands, repeat, as often as possible. Eventually the septum will break down and you won't require as much gripping force to get a good stretch.