This is such a simple question to ask, and it actually has a simple answer, but the answer has not-so-simple implications.

Many people want to answer the question with “setting”, but that is only partially correct.  Setting is “the time, place, and conditions in which the action of a book, movie, etc., takes place” (Merriam-Webster).  It is only the within the story you are writing.

But there are other things happening that do not take place in the time, physical space, or conditions of your story.  Things have happened before the story, they will happen afterward.  People exist outside of the story you’re telling, in places that exist outside of the story you’re telling.

It is these empty spaces, the ones you don’t tell us about in your work, that is world building.

World building is everything that happens in the reality of your story, both inside and outside of it.  And I mean absolutely everything.  No matter what world you create, people have to remove waste from their bodies.  How do the characters do that?  How does the society handle it?  This is a very simple aspect of world building, what you’re characters are doing, no matter who they might be, off-camera.

There is more to your world than your story.  What’s happening outside of it?