This page contains information regarding Star Trek: Discovery, and thus may contain spoilers.
The mycelial network was a discrete subspace domain containing the mycelium, or roots, of the fungus Prototaxites stellaviatori. The network could be conceptualized as a vast microscopic web, an intergalactic ecosystem, or an infinite number of roads leading everywhere. It spanned the entire multiverse that included the known universe and all other quantum realities. (DIS: "Choose Your Pain", "Despite Yourself")
Its matter inside the network was different from that in normal space. The JahSepp were a species that specialized in breaking down all matter that entered the mycelial network to make it useful in new ways. (DIS: "Saints of Imperfection")
The intersection between the mycelial network and normal space was called the mycelial barrier. A ship utilizing spore drive existed inside the mycelial network for a few miliseconds during a jump. A partial jump on a predetermined course with no set end point could leave the ship half in, half out of the network, the ship functioning as a doorstop. While inside the network, mycelia would attack and decompose the ship, although it took some time to eat through tritanium. (DIS: "Saints of Imperfection")
The network could be accessed via the mycelium spores of P. stellaviatori. At least two technologies were based on it, the Terran Empire's super-mycelial reactor, and Starfleet's Spore drive. Additionally, lifeforms such as the "tardigrade" could travel through the network naturally. (DIS: "Choose Your Pain", "What's Past Is Prologue")
Those traveling along the network might find themselves near-instantaneously transported over long distances, or to the same coordinates in a different universe. While traveling to another universe, there was a risk of "overshooting" and arriving months later than intended. (DIS: "The Butcher's Knife Cares Not for the Lamb's Cry", "Despite Yourself", "What's Past Is Prologue")
While experimenting with spore drive, USS Discovery found pockets of negative mass within the data it accumulated. The crew theorized that these could indicate alternative parallel universes connected to the mycelial network. However, the exact coordinates of some of these more esoteric destinations eluded the ship, until Lorca managed to fill in these gaps using a highly complex jump pattern. With these coordinates, Discovery attained the ability to cross into the Mirror universe. (DIS: "Into the Forest I Go", "Despite Yourself")
2250s threat EditMirror universe, Paul Stamets utilized the mycelial network in order to build a super-mycelial reactor. However, the technology had a major flaw: its orb element pulled power directly out of the network in a way that prevented it from regenerating itself, and in fact poisoned it, spreading infection back up into the mycelial system.
This caused the network to deteriorate across the entire multiverse, in a manner which might eventually have proven irreversible. Effects of the corruption included the dying of Prototaxites stellaviatori. If the damage was allowed to continue, life as we know it across all universes might have ceased. (DIS: "The War Without, The War Within", "What's Past Is Prologue")
The crisis was seemingly averted when the USS Discovery managed an attack on the ISS Charon, the ship on which the super-mycelial reactor was installed, and severed its orb from the network. The network began regenerating immediately. (DIS: "What's Past Is Prologue")