There are a lot of versions of the story, depending on who's telling it and where you are. They are all slightly different, but the core of it remains the same. Long ago, before the deaths of the gods, they dallied with mortals. Sometimes with an animal or something, but most often with Man. These indiscretions often led to singular men and women. Mortals with the powers of the gods. As time went on these people became leaders of men.
After the fall of the gods many people were born with power; people who were not part of the lineages that already existed. This was a time of great upheaval. Many old regimes were over thrown and the world fell into a dark age. Then came Gretta the Great and the birth of the Heilige Königreich. Gretta was able to do this by tapping into a new level of power. She became the First High Sorcerer, and the First Empress of what would eventually become Diutisca. She began gathering up as many of the nascent Sorcerers as she could find, founding the first Sorcerous Lineage. Eventually many other countries did the same. It was the only way to compete. The rule of Sorcerers became necessary in order to remain an independent country or city state. It has been centuries since teh rise of Gretta and her Heilige Königreich. The nobility had become decadent and corrupt. The Final War and teh Spell Plague changed everything. One in three nobles died, now each of them is more important to their house than ever. Many of the common people blame the nobility for the Spell Plague and the horrors that sprang from it.
Everyone has that one thing that they are really good at.A skill or a talent they can do better than most of the people around them. They might not be the best in the world, but when it comes to this talent they are pretty good. At its very weakest form High Sorcery lets the sorcerer be better at her particular skill than anyone on the planet save for another High Sorcerer. At the weakest form, these nobles have a gifting like unto a demigod. They can be as strong as Heracles, musical talent like Orpheus, the speed of Hermes, or the mind of Odysseus. These are just inborn gifts. There is no training required. If your mother had the gift you will too.
Where it gets weird is when the sorcerer gains followers. The more people the sorcerer has dominion over the more power the sorcerer has. The first and easiest form of this additional power is an increase in the Sorcerer's already impressive skill. Secondly The Sorcerer will be able to tap into the skills of her followers. This means that if the followers have a skill the Sorcerer will have the skill at the highest level of all her followers. The last ability granted by followers is a little strange. The Sorcerer can temporarily shore up the abilities of her followers. Basically for a scene the Sorcerer can lend her power to all of her followers. Want to win a war have a Sorcerer with a gift for fighting lead the army. Mind you that army will now be her's, but it will be nearly invincible in combat. Sorcerers are absolutely one of the most terrifying beings in the world.
Sorcery is passed through the maternal line. If your mother was a Sorcerer, then you will have the gift. Women in general have an easier time using Sorcery. Most men can never get past the first level of sorcery(gifting). This still makes them terrifying to common men, but it is the rare man that can master all three levels of Sorcery. Sometimes a new sorcerer will be born to a family with no history of the gift. Usually the nobility quickly and quietly move that family into the nobility and hope no one notices. Using propaganda and bribery, few people ever do notice. Being born the Daughter or son of a female Sorcerer is a guarantee of a gifting, but not of what the gift will be. the noble houses have a tendency to bear children of a specific type of Sorcerous Gift, but they still get children of different giftings. No one quite knows why this is. Most attribute it to the fickle winds of Fate.