Answer:
It is the place where most of the story takes place. For example, if you are writing a story about far-away aliens, the primary setting will probably be outer space.
Step-by-step explanation:
Primary basically means "main", so you can infer that "primary setting" means the main setting of a story.
Please keep in mind that I have never heard of this term before, so I'm really just guessing. However, I do believe this is the most likely meaning.