"Great achievement is usually born of great sacrifice, and is never the result of selfishness."
"The amateurs discuss tactics; the professionals discuss logistics" -Napoleon Bonaparte
The Astronomican is a psychic marker pen the Emprah used to mark his patch of the galaxy, which is basically all of it.
The Astronomican is a device created by the God-Emperor of Mankind in preparation for the founding of the Imperium of Man. Following the Golden Age of Technology, humanity was cut off from its colony worlds due to violent Warp storms and the loss of its own beacon grid, rendering it unable to safely traverse the Warp. While the Warp storms died out around M29, it was all but impossible for Navigators to safely guide space ships through it. To solve this, the Emperor created the Astronomican, a device which acts as a psychic "lighthouse", allowing warp-travel in a 50,000 light year radius of Terra. The Emperor eventually planned to phase out Warp travel completely with the construction of an Imperial Webway, but one guy accidentally ruined it, which led to an immeasurable amount of rage due to Not as planned, both from Big E. himself and the fan-base alike.
Following his entombment to the Golden Throne, the running of the Astronomican fell to the Adeptus Astronomica. It's overseen by the Master of the Astronomican, who has a permanent seat on the council with the High Lords of Terra. Given the sheer amount of psychic power contained within the device, the Emperor's mind is used to direct the power of the Astronomican. To keep it running, one thousand psykers are rounded up every day and sacrificed to power the Astronomican. Unlike the ones sacrificed to maintain the Golden Throne,
these psykers are trained to keep it running and consider it a holy duty to sacrifice their lives *BLAM*, Imperial fanatic propaganda. The device is a trash can for psykers that are weak of mind, outright insane, or just happened to piss someone off. By keeping it active, humanity has a guaranteed way of navigating the Warp, "depositing" dangerous psykers and keeping the Imperium "functioning".
There's actually way more than one thousand psykers inside the Astronomican Chamber of the Hollow Mountain, the former Mount Everest. The mechanisms of that chamber gradually drain each victim of all memories, thoughts, emotions, and eventually the soul itself through a course of a few weeks, or in case of the most unfortunate ones, months, and all this time for them is filled with extreme, incomprehensible, mind-blowing, "I wish I was abducted by the Dark Eldar" pain, until all that is left of their souls ceases to exist (so no afterlife for them) and all that is left of their bodies crumbles to dust.
It lights up the warp, Council-of-Nikaea-beacon style, filling minds of psykers with holy singing, thoughts of martyrdom and images of Living Saints of course.
The Talon of Horus book by Aaron Dembski-Bowden describes... interesting side effects of the Astronomican activity. Where its light collides with a potent Warp Storm, like the Eye of Terror or the Maelstrom, the mix of both creates the Firetide : a rim of space fucked up so hard even daemons are afraid of it. Not only do they get quickly burned by its walls of holy warp-fire, these are also inhabited by warp-spirits that could be described as flaming angels; just like Chaos daemons, they don't mind slaughtering, raping, and eating chaos worshipers (who, if they are very lucky, will suffer this fate in that order). These "angels" are known to cleanse their territory of all life oldcron-style, no matter its allegiance, but unlike daemons, these spirits cannot be reasoned with, as they are as insane from millennia of intolerable pain and suffering, probably just like the source that gave them life.
The Firetide engulfs multiple "Radiant Worlds", planets touched by the golden psychic storm without being burned by it.
In these regions, the Astronomican can manifest an avatar of the Emperor's will known as Imperious, an actually way nicer chap than these postal-going "angels" described earlier, taking the form of a humble pilgrim wearing a Scream mask of gold and light.
There's only one real drawback to having the Astronomican (you know, aside from whole sacrifice of a thousand innocents per day): it's attracting the Tyranids to Terra (they were already on their way to the Milky Way due to the Pharos) like a swarm of mosquitoes to an infra-red torch so it should really be renamed to the Astro-NOM-ican. Is this a drawback? Who knows? Maybe Big E. needs the 'nidz to set him free from his Golden prison. After all, their presence would likely close the warp rift in the Eternity Gate as it is so small and their Shadow in the Warp is so large. While sacrificing the trillions upon trillions of humans on Terra. Typical dickery- though on second thought, there are many examples.
The fact that the psykers are sacrificed and are not allowed to rest between sessions is also a typical source of grimdark.
That Other Thing
It's not mentioned much, but the Adeptus Astronomica is the single most important organization in the entire Imperium. Without it, there is no Imperium. Humanity is only able to leverage its numbers and make war on a galactic scale because the reliability of its warp travel. Yes, sometimes ships are lost to the Warp or arrive two centuries too late. However, sometimes every successor chapter answers a call and arrives to bail out Baal or Terra all at once. Sometimes crusades of hundreds of ships go cleansing a path hundreds of star systems wide. No other race on the galaxy has the numbers, reactiveness and coordination, and it is owed to the Astronomicon.
Also warp travel being what it is sometimes it fucks up in a beneficial way and the fleet arrives several years before it left.