What does that mean?

https://jsonapi.org/format/#fetching-relationships

I still don’t understand. based on the image, if I REQUEST to url ‘/articles/1/relationships/author’
2 times, then it will return null as the main data?

It will either return { "data": null } if the post does not have an author or the resource object representing the author of the post. For the first example that would be the people record with ID "12".

Assuming that data does not change, it doesn’t make a difference how often you execute the request. A GET request should read current data only. It should not have any side effects.