The Prime Universe is the center of the 10,000 Dawns, and the source for which all of the variant universes surrounding it spun off from. While every universe in the 10,000 Dawns has it's own distinct set of events, they all in some way resemble or echo those in the Prime Universe, with many of the same people in the Prime Universe appearing in other universes of the 10,000 Dawns despite their diverging histories.
For a list of important events here, see the Prime Universe Timeline.