Choosing one strange course of action near the end of reveals a treasure trove of hidden lore, but doesn't explain it to the fullest extent.
Taking place in the Forgotten Realms setting for, incorporates 40 years of lore. For the most part, it's a complete story in and of itself — most central concepts in the story are well explained through dialogue.
However, it still exists in the context of millennia of Faerûnian history, and occasionally touches on a more arcane piece of background. [Warning: This article contains some spoilers for Act Three of Baldur's Gate 3.] Suffice it to say that has a huge body of lore.
While it's totally playable without engaging with any of this optional background, in-game history books and throwaway references reveal a much more complicated story.
Looking out for these references can enrich the experience, but it's not always clear what they're referring to. That's all the more apparent in a certain endgame line from everyone's favorite githyanki hero.