It goes without saying that such information is a major spoiler for the movie, so those who wish to avoid such information need to stop reading.
What follows is the information presented in the comic.
The Fallen and his "brothers" were born on a terrestrial world where the Allspark was located. They learned that the energies of the Allspark were limited, and could only be restored through the destruction of a sun.
In order to help them, the Allspark created workers who would go out and search for suns that would help to fuel the Allspark. These workers were the first Cybertronians. (Among which is Jetfire.)
The Fallen decides he wants the power of the Allspark for himself, but was trapped in his sarcophacus (the ancient relic discovered by the Transformers in Defiance #1), where he is to remain until the sun harvester is located and reactivated once more.
Given the amount of information previously revealed regarding certain elements of the movie's plot, it's safe to say that those wanting a complete picture of what happens now have it.
And that the Transformers universe, as crafted by Michael Bay, will never be the same again.