The rumors are saying that the character you play in Skyrim was born with the soul of a dragon, so he's a dragon in a mortal body which gives him powers, according to some prophecies, which is why you can absorb the souls of dragons. The Nords learned to use dragon shouts from the dragons in ancient times (possibly) but were banned from distributing that knowledge when they joined the empire, which is why the greybeards are the only nords with those powers and they have to stay on their mountain. Well, Tiber Septim was the first(?) emperor of the "dragon blood" and my theory is that he was a prophecied hero just like the hero you play in Skyrim, complete with a dragon's soul and voice powers. He kicked major ass, became emperor, and later ascended to godhood/was deified. His descendants are all called dragonborn but he was the only REAL dragonborn. All of this is a theory based on preexisting lore, and my assumptions based on what is revealed of Skyrim in the trailers. Makes since though right?