Subspace is the underlying domain of space-time that makes faster-than-light travel and communication possible, and whose properties shape much of starship technology.
Warp fields, long-range sensors and faster-than-light communications all depend on subspace, which also carries distress beacons and the signals of the Federation's relay networks. Starships send messages through subspace relays, detect distant objects by their subspace signatures, and form warp bubbles by distorting it. Many anomalies encountered by Starfleet crews, from rifts to strange life forms, exist partly in subspace.
Subspace can be damaged. Research in the Hekaras Corridor showed that intense warp travel could produce rifts, and certain weapons, such as isolytic charges and the Omega molecule, threaten it directly. Starfleet treats damage to subspace as a serious danger, because it can make whole regions impassable to warp-driven ships.
In the 32nd century, the Burn was traced to an event in a subspace-rich region of the Verubin Nebula, which caused most of the galaxy's active dilithium to go inert at once.