Time of Rebirth
The Time of Rebirth was the period of Imperial history that took place during the 31st to the 32nd millennia, right after the Great Scouring which in turn had followed the Horus Heresy and the second founding which divided the remaining Space Marine legions into smaller chapters to prevent a recurrence of the events of the Heresy.
The Rebirth was a time when the Imperium worked hard to get its shit back together following the upheaval of the Heresy, the internment of the Emperor into the Golden Throne and a whole lot of damage caused by the traitors on the young empire's worlds and people. Roboute Guilliman enacted sweeping changes to Imperial organization during the beginning of the Rebirth, not just with the Space Marines, but separating the Imperial Army and Navy and creating the Imperial Guard and Imperial Navy; as separate organisations with their own command structures they would be harder to corrupt if chaos tried its tricks again, as it would do countless times in the future.
As the period of repair continued, xenos species which had been pushed back during the Great Crusade reared their heads again and assaulted Imperial space; in particular the largest Ork Waaagh! the galaxy had seen at this point attacked many worlds and only at great cost to the new space marine chapters (including the extinction of the entire Imperial Fists, and the death of Vulkan(again)) was the Waaagh finally stopped.
Towards the end of the Time of Rebirth was the hideous event known as The Beheading, when the Grand Master of the Order of Assassins had the High Lords of Terra, who had taken over the reins of running the Imperium in lieu of the Emperor, killed to a man. After said Grand Master finally snapped from competently ruling the Imperium for nearly a century, a strikeforce of space marines tracked down the Grand Master to a temple where they fought a hundred Eversor assassins. A single marine managed to survive and kill the Grand Master and ended the beheading. The anarchy that followed this time as new High Lords rose to the power ended the Time of Rebirth and lead to the new chapter of Imperial history, the Forging.