What follows is just a brief note as a prelude to other posts.
If someone tells you about the staircase in their house you might reasonably ask how many stairs the staircase has. But when we read, In Little Dorrit "Mr. Flintwlnch, therefore, wormed himself up the staircase" it makes no sense to ask how many stairs this staircase has, just as it makes no sense to ask how much Hamlet weighs. This is because the fictional object is co-extensive with its description. It has no attributes other than those described.
With real-world objects there is always something about them surplus to description, something that can be questioned or explored. It is always meaningful to ask how many stairs a staircase has or how much someone weighs. Fictional objects cannot be questioned or expored in this way.
We might further say that a fictional world is, put simply, a narrative or description wherein the people, objects and events have no real world referent.
An objection might be:
Suppose a novel begins "On the 5th September 1949, he walked down Charing Cross Road'. Clearly this referes to a particular time in a real city. Fine, but we are still dealing with a fictional world because the events narrated in the novel did not in fact occur in London on that day. The actual real-world London of 5th September 1949 was not a London containing those events, and the novel does not therefore refer to that world.