Or, "Those things that people reference on occasion, usually to say how much they hate them."
This is optional, and tl;dr. Feel free to skim it. This just exists as a useful reference if you'd rather read a couple paragraphs that summarize the entirety of Dragonsdawn, Dragonflight, Dragonquest, The White Dragon, and All the Weyrs of Pern.
Thousands of years ago, a bunch of space colonists landed on a planet they designated P.E.R.N. - it stands for Parallel Earth Resources Negligible, a reference to the fact that it was hospitable to human life, but did not possess enough valuable resources to be commercially interesting to interstellar companies. But this little group of settlers wanted a return to a more peaceful, isolated, agrarian way of life, with limited technology. And they got what they wanted. Boy,
did they get what they wanted.
Shortly after the establishment of the first colony, they discovered that Pern was subject to periodic attacks by Thread, a spaceborne spore trailing in the wake of an interstellar body known as the Red Star. Whenever the Red Star passed too close to Pern it would shed Thread capsules into Pern's upper atmosphere. The capsules would then break open, raining down thin silvery filaments that devoured and destroyed any organic matter that it touched. This, naturally, kind of put a damper on basic necessities like farming. And living. The colonists discovered that either fire or water would break down the Thread, but they needed a better way to defend themselves during the frequent Threadfalls.
So they engineered themselves dragons. Flying, firebreathing, psychic dragons. They used a native species, firelizards, as a base, noting that the firelizards had both the remarkable ability to teleport (using a hyperspace dimension known as between
) and breathe fire by chewing a phosphine-laden rock, colloquially known as firestone. Kitti Ping, a geneticist among the colonists, engineered the first dragons to be large enough to ride and of human-equivalent intelligence. She also engineered them to be completely dependent on humans, needing to Impress to a single rider at birth and incapable of harming any human after Impression. This was a safeguard to keep a whole bunch of genetically engineered superpredators from taking over the planet.
Fast-forward thousands of years. The colonists have entirely lost all records of where they came from, and have reverted to a pseudo-feudal society centered around the periodic Threadfalls and the dragonriders that protect Pern. A whole bunch of bullshit happens with some riders named Lessa and F'lar that you don't really need to know about. The relevant
thing is the discovery of AIVAS, the original ship's AI that came with the first colonists. AIVAS re-taught all this history to the Pernese, as well as some limited advanced crafts - obviously no one's going to go from farming to building microprocessors in a generation, even with a supercomputer to help you, but some scientific advances are low-hanging fruit.
In addition, Lessa and her dragon Ramoth discovered that dragons could travel between
through time as well as space, albeit at a greater risk. With the assistance of a mutant white dragon named Ruth, and AIVAS' fantastic supercomputer brain, they successfully went back in time and altered the orbit of the Red Star (yes, I know that's physically impossible, it's in the books, bear with me) so that Thread would never fall on Pern ever again. And then everyone lived happily ever after.
Enter Nightwatch. From here, that's where our weyr diverges from "book canon".