No'Mat was a location in the mountains of the planet Qo'noS. In this location, there were lava caves. 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")
Years later, when "Kahless" returned on Boreth, Worf was convinced that he was the real Kahless when he narrated his appearance and words he had said to the young Worf. (TNG: "Rightful Heir")
Appendix
According to the Star Trek: Star Charts, on page 61, No'Mat was a planet in the Omicron Leonis system. This system was located in Klingon space, in the Beta Quadrant.