In Naruto, Susanoo is a giant being that fights for its user, Uchiha Itachi, in battle. It is an extremely rare result of learning a "jutsuu", or technique, called Mangekyō Sharingan.
Those who can control and use Susanoo are extremely rare. When Uchiha Itachi witnesses his close friend Shisui commit suicide by throwing himself off a cliff, an ability called Mangekyō Sharingan awakens in the forms of Tsukuyomi, Amateras and Susanoo. Susanoo is extremely powerful, fighting for and defending Itachi in battle. During battle, Itachi can freely move while inside Susanoo, walk outside of Susanoo and allow others inside to be protected. Using Susanoo comes at great cost, as Susanoo uses Itachi's chakra, or energy, to materialize and fight in battle. Because Susanoo is using Itachi's life energy, it causes him to be in tremendous pain and shortens his lifespan.