This page contains information regarding Star Trek: Discovery, and thus may contain spoilers.
The reaction cube; also called the spore cube, was a test chamber aboard Crossfield-class starships. On the USS Discovery it is located in the engineering test bay alpha. (DIS: "Context Is for Kings")
Before the spore drive was perfected, Captain Gabriel Lorca explained to Michael Burnham the concepts behind the mycelial network by injecting mycelial spores into the reaction cube while she was standing inside of it, showing her different locations across the galaxy. (DIS: "Context Is for Kings")
Burnham and Paul Stamets later determined that the Glenn's spore drive was experimenting with using a tardigrade in the reaction cube to guide the spore drive's navigation. (DIS: "The Butcher's Knife Cares Not for the Lamb's Cry")