No'Mat was a location, known for its lava caves, in the mountains of the planet Qo'noS. These caves played a role in the Rite of MajQa wherein a young Klingon while meditating would experience a vision. This vision, or hallucination depending upon one's interpretation, might play a role in the Klingon's future. When Worf was young, he ran away to the Caves of No'Mat and had a vision of Kahless, and was told that he'd do something no other Klingon had ever done. (TNG: "Birthright, Part I", "Rightful Heir"; DS9: "The Sword of Kahless")
It was not explained in the episode how the clone of Kahless knew of that event.
The Star Trek Encyclopedia (3rd ed., p. 325) also stated that No'Mat was a planet.
According to the script for "Birthright, Part I", this location's name was pronounced as "no-MAT".