The lore of Terraria refers to the game's back-story.
While Terraria was initially released without a clear story component, its developers at Re-Logic wrote and released a story connecting its characters and gameplay elements in celebration of the 8th anniversary of Terraria's release. The developers have mentioned that this is intended to be one possible interpretation of the lore, and that players are encouraged to piece together a story (or lack thereof) as they see fit.[1][2] Regardless, many entries in the Bestiary contain matching pieces of lore.
The story was released on May 16th, 2019 as images of inked parchment, via the official Terraria forum.