. The basic premise - if, for example, a scientific idea or a detail of history is important to the story.
. World building - if the physical character of your world is important (an unusual or non-Earthlike planet, for example); or if your world-building is drawn from mythology or some source other than your own imagination.
. Regional language and geography - if you're setting a story in Boston, there will be details that are very different from a story set in Caracas.
. Details to make your scene believable - whether your characters are riding a horse, loading a musket, felling a tree, fueling a rocket, diving to the bottom of the ocean, or surveying a black hole, you should do your best to get the facts right. (Some of it's guesswork, of course. No one has ever orbited close to a black hole, and there's a lot we don't know about them; but there's a lot we do know about them, and you need to take that into account before you start making up the additional details.)