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")
According to the Star Trek: Star Charts, on page 61, No'Mat was a planet in the Omicron Leaonis system. This system was located in Klingon space, in the Beta Quadrant.