Nurn was a region in the south of Mordor. The great Lake Núrnen, which was fed by four rivers, was located in that region. Núrn was much more fertile than Gorgoroth, a literal desert, because it was able to sustain fields to grow food.
At the time of the War of the Ring, slaves worked fields by the dark, sad waters of Lake Núrnen to feed the armies of Sauron.
The region of Nurn most likely escaped the desolation caused by the eruption of Mount Doom because it was much further away from Mount Doom than Barad-dûr or Udûn.
After the War of the Ring, King Elessar of the Reunited Kingdom of Arnor and Gondor liberated the slaves of Mordor and gave them all the land around Lake Núrnen to be their own.
