From the publisher's website:
In the Imperium of Man, technology is commonplace...but it is also mysterious and unfathomable. Such knowledge is the carefully guarded domain of the Adeptus Mechanicus, and has been for thousands of years. They are the Cult of the Machine God, worshipping the Emperor as their Omnissiah, the Perfect Sum of All Knowledge. Their Explorator fleets search across the galaxy for lost knowledge from ages long past. Their mechanically augmented tech-priests on thousands of forge worlds strive to preserve what precious little data they retain. Both quests continue, though they are mere flickering candles against the oppressive darkness of stagnation and ignorance. With each century, actual understanding descends to dogma, rote instruction, and myth, as priceless knowledge is lost forever.
Within the Calixis Sector, the Adeptus Mechanicus is centered on the Lathe Worlds, and from these three planets the Mechanicum oversees its domains of forge worlds, research stations, orbitals, and even entire systems. From these worlds, precious knowledge and holy technology flow throughout the Sector, while the Cult Mechanicus carefully hoards even greater secrets for its own, inscrutable purposes.