The independent pilots who carried goods between isolated worlds after the Burn made dilithium scarce, working out of mercantile exchanges and often at the mercy of the Emerald Chain.
Couriers are introduced in the third-season Discovery premiere That Hope Is You. With warp travel crippled by the Burn, trade depends on freelancers with their own ships and whatever dilithium they can obtain, who take jobs at exchanges such as the Mercantile on Hima, run by Orion and Andorian syndicates.
Cleveland Booker is a courier who uses his ship to smuggle endangered trance worms to a sanctuary world. Michael Burnham, arriving alone from the 23rd century, falls in with him and spends a year flying as a courier herself while she searches for Discovery, as shown in People of Earth.
The trade is dangerous and often unfree. In Scavengers Burnham and Georgiou rescue Book from an Emerald Chain salvage yard on Hunhau, where captives are held as forced labour.