Warning: contains spoilers for The Death of Doctor Strange #1!
Marvel explains the true nature of Doctor Strange's classic title of Sorcerer Supreme. In the Marvel Universe, Stephen Strange has held the title ever since Doctor Strange #10, when his mentor the Ancient One died (only physically). Now in The Death of Doctor Strange #1, written by Jed MacKay with art by Lee Garbett and colors by Antonio Fabela, none other than Strange himself gives a detailed explanation of the title - and responsibilities - of the Sorcerer Supreme.
Doctor Strange is one of the most powerful superheroes in the Marvel Universe, able to cast spells and defend against the most ferocious of otherworldly creatures. While he often fights alongside the Avengers against more earthly foes, his specialties lie in battling enemies from other realms and dimensions, such as the classic Strange villain Shuma-Gorath and Dormammu, ruler of the Dark Dimension. But his title as the Sorcerer Supreme has rarely been elaborated upon; in The Death of Doctor Strange #1, it's more than a ceremonial title or name that Strange gave to himself.
During an ordinary day in the life of Doctor Strange (which would unfortunately be his last), Strange pays a visit to the Strange Academy in Louisiana and gives a lecture to the assembled students. He explains that the title of Sorcerer Supreme was granted to him by the Vishanti - a trio of powerful, extradimensional entities and defenders of the Earth dimension - and was thus selected to protect the Earth; they've recognized his superior skills as a magician. "As Sorcerer Supreme of the Earth dimension, I am granted access to great wellsprings of power unavailable to other practitioners of the mystic arts," Strange explains to the class. But with a superior title come superior responsibilities.
Strange then explains about the barrier: a master spell that strengthens the boundaries between Earth and other potentially hostile dimensions that wish to invade the planet. Maintaining the barrier is one of the many duties of the Sorcerer Supreme. Marvel has previously hinted that the title was merely ceremonial in nature (while Doctor Voodoo held the position), but there is discernible power offered to the Sorcerer Supreme. Furthermore, the position is not permanent - if Strange fails in his duties (or if he is killed), the title of Sorcerer Supreme will be offered to another. Strange says as much in the classroom: "My tenure in this position will not last forever."
Strange's words are unfortunately prophetic; he is killed at the end of the issue by an unknown assailant. It appears Marvel seeks to clarify the nature of the title of Sorcerer Supreme before choosing another to take Strange's place. Now that Doctor Strange is dead, the powers of Sorcerer Supreme must pass to another - but as to who the mystery candidate is, only time will tell.