Dilithium is the crystalline mineral used to regulate matter and antimatter reactions in warp cores, making it one of the most strategically valuable resources in the galaxy.
Dilithium is uniquely able to channel the energy of matter and antimatter annihilation without being consumed by it, which allows a warp core to run in a controlled way. Because ships cannot travel efficiently at warp without it, sources of the crystal have long been prized, traded and fought over.
The Enterprise under Captain Kirk often depended on scarce supplies. A diplomatic mission to Troyius turned on that planet's dilithium deposits, which the Klingons coveted, and in the mirror universe the brutal Terran Empire threatened the Halkans to seize their crystals.
By the 32nd century dilithium had become even more precious. The event known as the Burn caused most active dilithium to go inert at once, destroying many ships and leaving the remaining supply under the control of whoever held it. Starfleet's scattered remnants and rival powers such as the Emerald Chain competed fiercely for reserves.