Dragonborn (Lore)

In the Elder Scrolls universe, the term "Dragonborn" is used to refer to the descendants of Emperor Tiber Septim, founder of the Septim Dynasty of Tamriel, who himself was a descendant of the main Tamrielic Aedra, Akatosh, and St. Alessia, who founded the Cyrodiilic Empire in the First Era and eventually led to the Septim bloodline.

Tiber Septim as "Dragonborn"
Tiber Septim, who ushered in the Third Era of the Empire of Tamriel, is the only known mortal to truly ascend to godhood. (The player-character's actions in The Elder Scrolls IV: The Shivering Isles should not be considered true apotheosis, as they are still a mortal despite "wearing the mantle of a god".) Upon his ascension, Tiber Septim became Talos, the Ninth Aedra and God of War to the followers of the Nine Divines. However, his bloodline continued i the realm of mortals through the Septim Dynasty, up until the events of The Elder Scrolls IV: Oblivion, where Martin Septim, the last member of the Septim bloodline, sacrificed his life to stop the invasion of Tamriel by the Daedric Prince of Destruction, Mehrunes Dagon.

"Dragonborn" in Skyrim
"But... there is one they fear. In their tongue, he is 'Dovahkin-- DRAGONBORN!" -Narrator in the first Skyrim trailer.

In the first teaser trailer released for Skyrim, the narrator mentions the apparent reawakening of the Dragons. Immediately after, he mentions that they fear the one they call "Dovakhin"-- Dragonborn. What significance this person shall play in the upcoming game is currently unknown.