The Void Which Binds, more commonly known as Planck Space, is an invisible and intangible domain which permeates the known universe; as its name suggests, it is the plane which unifies all of existence, unlimited by space and time. While it's precise nature is poorly understood, it is used by various entities in the universe for travel, communication and even habitation.
It is suggested numerous times in the Cantos that the Void is the place where every consciousness is located, as a quantum wave of probabilities. The human emotion, reasoning and self awareness, as well as other sentient species', are projected in the Void. The physical location of those other races and their identity is never deemed clear throughout the Cantos, but it is said that some of them are organic and some artificial, and that they inhabit both our galaxy and others. They posses ultimate control over the Void, and are therefore capable of manipulating space and time. This feat enables them to experience past, present and future events, as well as casting physical substrate throughout the universe. The energy to power their projects comes, it is claimed by Aenea in The Rise of Endymion, from quasars and blasting cores of distant galaxies.
The TechnoCore used the Void to manufacture extremely powerful innovations which they shared with humanity in the time of the Hegemony, the most prominent among which being the Farcasters and Fatline communication. Those technological advancements were beyond the understanding of the Hegemony scientists who tried to reproduce them, since they all actually worked through the Void, and therefore were inaccessible by other means.
These wonderful artifacts were not truly gifts, however: The Technocore used the farcasters to absorb neural computational power from the brains of the billions of humans using them each second. To any person using a farcaster, it appeared that no measurable time elapsed between entering and exiting; however, because the TechnoCore exists outside of normal time, they were able to take advantage of the infinitesimal transit period to "dissect" and analyze the brain of the unsuspecting human.
The Void connects all the consciousnesses in the universe. It is no wonder, then, that true control over the Void can be achieved with empathy and love. Those who master the Void are capable of binding their consciousness with the consciousnesses of other creatures, living or dead, thus accessing their memories and thoughts, as well as performing freecasting - the ability to freely travel through space and time by means of the Void.