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.
The Death Star. The fact that the Empire built an incredibly advanced, moon-sized weapon of mass destruction not as a tool of conquest but as a means to deal with internal threats tells you everything you need to know about them.
And it works in so many other ways - it strengthens the WWII motives (referencing Hitler’s obsession with oversized superweapons) and is of course a major plot source to countless stories, from the harrowing tale of the guy who pulled the superlaser trigger to the plucky rebels who stole the plans. No wonder they keep returning to it lol