Tellun was a prison colony where convicts were employed as miners. Half the "screws" were said to be Andorians.
In 2256, there was reportedly an accident in which a dilithium pocket went piezoelectric and ripped apart the bottom of the mine. Fifty miners were said to have been vaporized.
Four prisoners, including Michael Burnham, were being transferred to the mine by prison shuttle, when it was rerouted to meet with USS Discovery. After the shuttle encountered Species GS54 near the ship, the prisoners had to be temporarily taken aboard. (DIS: "Context Is for Kings")
The phrase "on Tellun" was used, suggesting that it was some kind of world (or asteroid), and not a direct reference to the Tellun system, although that may still be related.
According to StarTrek.com, the convicts were being transported to the planet Elas in the Tellun system. This planet, under Federation control, had large dilithium deposits which were mined by the convicts. [1](X)