Who asked me to repost, so here it is
Who are "they?"
Most obviously, dragons. However, this is not exactly a satisfactory explanation. The Tamrielic dragons, like Nafalilaargus in Redguard, were not defeated and nothing in lore suggests that they would be coming back in a dramatic, dangerous fashion. We also have no historical record of them ever terrorizing Tamriel. If they really are just strait up dragons, it could be a retcon or a gap in our lore knowledge. Other possibilities include the more metaphorical:
Tsaesci/Akaviri - they have been defeated in the past, and their coming back would be dangerous and dramatic. Dragons are also said to originate from Akavir. However, Akavir is not exactly near Skyrim. I am also unsure as to why the Akaviri (well acquainted with Dragons, Immortal Vampire Snakemen, and God-Tiger-Dragons) would be afraid of the Dragonborn. Their existence is not exactly disputed, either.
Akatosh - the Dragon Lord of Time, and Nordic Alduin, that is "World Eater." He is the destroyer of worlds, and perhaps the Nord's biggest 'enemy' (as much as a Shiva-like god can be an enemy). "They" being multiple makes this less plausible, though it could refer to his multitude of underlings (including the Jills, or time-mending lady dragons). I doubt that the Nords don't believe he existed.
The Empire - dragons are their symbol, and they were defeated, and are set to come back. The sheer amount of gravitas granted to this event makes this an unlikely explanation. Again, their existence is not disputed. However, the nature of their divine right to rule (their dragon status) probably is.
It could of course be some sort of combination of these, like an Akaviri Emperor who is mantling Talos. Or It could be someone we've never heard of before.
Who is the Dragon Born?
"Dragon Born" is used to refer to the Cyrodil Emperors, due to their being 'born' from the Alessian covenant with Akatosh. While the Septim empire has fallen, we know that at least one (that of Titus Mede) has taken its place since. The Imperial armor on the man makes Imperial origin almost certain. Dragons are also associated with Akavir and with time, so "Dragon Born" could could be Akaviri or an avatar/embodiment/child of Akatosh (like a Shezarine, but with Akatosh).
Tiber Septim (anon Talos, anon Ysmir) is the emperor most often associated with the dragon sigil. He is also the patron of empire and conquest.
His being shown strongly suggests that the player will once again take a supporting role in TESV, since the hero (the Dragon Born) is established as male and mustachio'd. It is also possible that the Dragonborn will be reincarnated in/ mantled by the player.
The latest lore on Skyrim can be found in the Aldudagga, by Michael Kirkbride.
The teaser site gives us further details on the stella. Here are my additional thoughts: