This page contains information regarding new Star Trek material, and thus may contain spoilers.
The Mycelial network was a feature of the cosmos. It could be conceptualized as a microscopic web that spanned the entire cosmos, an intergalactic ecosystem, or an infinite number of roads leading everywhere.
It consisted of spores, which were the progenitors of panspermia and the building blocks of energy across the universe. Ships using spore drive could travel vast distances near-instantaneously by "jumping" across the mycelial network. (DIS: "Context Is for Kings")