Every living being has two opposing forces within them, forces which we refer to as Light and Dark.
Though most people can only feel one force, Taijitu is able to tap into both. Thanks to her training of Taijiquan, Taijitu is able to tap into these forces and gain abilities to defend and attack.
If the forces build high enough, she can divide herself into two seperate entities; Tai, essence of light and Jitu, essence of darkness.