Boreth was an inhabited planet in the Klingon Empire, and the location of the important Boreth monastery. According to the Story of the Promise, Kahless would one day return to lead the Klingon people on that "point of light". The planet later became a popular destination for devout Klingons seeking to discover the spirit of Kahless. To the Klingons, it is the most sacred location in the Empire.
In 2369, Worf visited the monastery on Boreth in an attempt to rediscover his Klingon heritage. It was there that Kahless apparently returned from Sto-vo-kor, although it was later revealed that he was a clone. (TNG: "Rightful Heir")
Dialog from the episode suggests that the monastery was about twelve days by shuttle from the general region in which the Alawanir Nebula was located, and a day's travel from the Klingon homeworld using the USS Enterprise-D. It might also be near the Gariman sector.
According to the script for "Rightful Heir", the pronunciation for Boreth was "BOR-reth".  However, according to the script for "The Way of the Warrior", the pronunciation for this word was "BOR-eth".  Finally, according to startrek.com, the pronunciation was "bore-OTH". 
According to the Star Trek: Star Charts (page 62), Boreth was located in the Boreth system. This system was a single star system. Primary was a Class M star with a magnitude of +3, which was 10 times brighter than Sol.
The novelization of "The Way of the Warrior" had several scenes set on Boreth.
The Bajoran Ascendancy, from the horrific alternate future of the Star Trek: Deep Space Nine - Millennium trilogy, named a Klingon warship after the planet.
Taking place in 2409, Boreth is also featured in the video game Star Trek Online. Alongside the clone of Kahless, the player has to fight demonic forces of the Fek'Ihri in the lava caves of the planet.