Lore
Also said as Backstory, Mythos
Everything known about the world beyond the immediate plot: its history, gods, wars and customs. Players usually meet it in pieces and mostly optionally.
Lore rewards the players who want more without holding up those who do not. It becomes a problem when you require it, or when you write it instead of making the game.
Say it
“Keep lore optional: nothing on the main path needs it, and every entry should be short enough to read in the time it takes to pick up the object.”
Where you’ve seen it
Dark Souls tells almost none of its history directly. Fans pieced it together from item text and dialogue, and arguing over it became a large part of how the game is enjoyed.
Not to be confused with story
The story is what happens during the game. Lore is everything that had already happened, and the player can miss it and still finish.