House of the Dragon on HBO is bringing us back to Westeros, but not to the Westeros most people are familiar with, at least on screen. The new Game of Thrones prequel series is set almost 200 years before the events of the original series, and that means a completely different set of characters and events to learn about.
The show is based on George R.R. Martin's book Fire & Blood, which serves as an in-universe history of the Targaryen family in Westeros. There are a few things and pieces that might be useful to you when you watch House of the Dragons first episode.
Who is King Jaehaerys?
Jaehaerys I Targaryen was known as the Old King. He sat on the Iron Throne before Viserys and played a significant role in setting the precedent of male succession in Westeros, when he allowed the Great Council of 101 AC to name Viserys his heir rather than his older daughter, Rhaenys. He left his son Viserys with a lot to live up to.
Aegon the Conqueror and Daenys the Dreamer, who are they?
Both have died for many years before the beginning of the House of the Dragon, but you may still think of them as the ones who initiated the Westeros' Targaryen dynasty, which is why they are still relevant to this story.
The Targaryens are from the Valyrian Freehold, a region of Essos on the Valyrian Peninsula. Their decision was made after a vision his daughter Daenys had of a catastrophic event that would have devastate Valyria forever. Twelve years after the family moved, she was proven correct.
Aegon I Targaryen set his sights on Westeros with a goal to conquer the continent and unite its various conflicting kingdoms and factions, roughly 100 years after the Targaryens moved to Dragonstone. After all, Aegon's assumption proved correct and he was able to unite all of Westeros (except Dorne).
Do the Targaryens always marry family members?
Targaryens married family members in order to keep the line of their bloodline strong. However, the royals have often married outside of their family. This is especially true of those Targaryens who are not likely to be heirs to the Iron Throne.
In its first episode, House of the Dragon, we see Rhaenys Targaryen (no, not Aegon the Conquerors' sister), a family that hailed from Valyria, but left the Freehold long before the Targaryens, lacking their dragon-riding ability and standing within the region. Despite this, house Velaryon has always been a key partner of the Targaryens, and he bears the nickname The Queen Who Never
The Targaryen kings still marry their children nearly 200 years after House of the Dragon. King Aerys II, also known as The Mad King, was married to his sister Rhaella.
Before watching the program, you may want to ask George R.R. Martin his own question in the video preview he created for House of the Dragon.