The Bleed is the anti-matter that fills the space between the realities of the Multiverse, separating them.
The Monitors can weaponize and use the Bleed for their own advantage as there is a special breed of them called the Weavers, who have been trained since birth to manifest it as a weapon. They use it to end all existence so they can maintain order. They have also reprogrammed some Manhunters, who are running on Bleed, to deal with the "supers" and "metas" of each Earth.
Even though the regular Bleed, also known as the Red Bleed, destroys everything it comes in contact with, immediately erasing it from existence, what is called the Blue Bleed can break everything down for reconfiguration or more simply reboot it. For instance when the Blue Bleed comes in contact with air or water, it breaks them down into base elements.
Years ago, Kryptonian scientists sought to explore space and travel the stars but during their study they discovered something adjacent to the main reality, the Bleed. The scientists were never able to explore it because it would seem nothing that nothing could come in contact with the Bleed, without being destroyed. The scientists eventually abandoned the project, fearing a possible breach from the "Bleed space", which contains all the parallel Earths that had ever existed.
In Antarctica, Lois Lane is touring a Higgs Boson supercollider facility. When the scientists start the experiment, a violent gateway opens pulling everyone and everything into the Bleed, the rift between parallel universes, with Lois clinging to a railing. Superman hears Lois call for help and flies to Antarctica at super speed. He arrives in time to catch her just before the vortex pulls her in, but they're both caught in the rift just before it closes.
On a parallel Earth, Superman and Lois emerge from the wormhole. Later, Superman is fighting with some Manhunters, when they suddenly evacuate in preparation for the "purge." Superman flies with Lois, trying to escape the Bleed that is destroying everything in its path. Superman hopes that the Fortress can send them back to their Earth. There, Jor-El teleports everyone somewhere else, moments before this Earth gets devoured by the Bleed.
After that, Superman and Lois wake up on another parallel Earth. There, Darkseid helps them board on one of the Monitors' ships flying overhead. While Superman carries Lois and follows the Monitors' ship into the atmosphere, Darkseid fires his Omega beams towards the ship, opening its side door so Superman and Lois can get inside it just before the ship enters the Bleed. Once inside, they lay low while Monitors investigate the duo's forced entry, unaware that stowaways have entered the ship and fearing of Bleed contamination.
In the Bleed space, inside the Monitors' ship, they recognize Superman as the person who killed Ray-Lan, so they decide to sentence both of them to death so they won't be able to interfere again in their plans. They intend to kill Superman and Lois by jettisoning them into the Bleed, as no one can survive it, not even the Monitors. Superman allows himself to be put into a chamber, which the Monitors flood with Bleed. A tearful Lois watches him being expelled from the Monitors' vessel. However, despite the rigors of Bleed Space, Superman manages to return to the ship and force his way back in.
Lois gets aboard a lifeboat and escapes from the Monitors' ship but just before she leaves a wounded Superman appears and gets away with her. As they travel through the Bleed space they see an Earth, that feels like home.
After Superman and Lois escape back to their home a Monitors' ship follows them and arrives at Washington D.C., beginning the attack using blue Bleed.