This page contains information regarding Star Trek: Discovery, and thus may contain spoilers.
The Mycelial barrier was the intersection of mycelial space and normal space.
Starfleet's spore drive utilized a reaction cube, which was the only place on the ship were it was safe to contact the mycelial barrier. Crew touching the barrier elsewhere would find themselves twisted around the axes of both environments. An accident of this nature killed the crew of the USS Glenn. (DIS: "Saints of Imperfection")