For some people, it’s a fictional technology that is detailed down to the very nuts and bolts. For others, a fictional culture that has all its elements seamlessly knit together to create a complex tapestry. A history that deftly tells the story of a person, nation, or planet, or an otherworldly species that feels real enough that it could exist, if only in another world.
What is it for you? What examples in fiction stood out for you? Why did they do; what about them spoke to you so strongly? It could be widely-known published fiction, or some niche project you ran into on the internet once.
For me its less so about the world-building itself but the presentation. I like when a fiction setting feels otherworldly like it operates on its own internal logic that makes you feel like a stranger in a strange land. One of my favorite ttrpg settings is vaults of vaarn, an indie game very much inspired by Caves of Qud and while the lore is fairly sparse, what you do get paints a vivid but weird history of an far future post-human earth. I think its neat.