World building only applies if you're creating a fictional world. That world could exist alongside the real world, or it could be entirely new. For example, a science fiction story on another planet might require you to describe who lives on the planet, when they populated it, what it looks like, if any aliens live there, and if so, how the aliens name themselves, reproduce, govern themselves, obtain resources or use currency, how long they live, how they typically die, how long they'be been in existence, where they originated, whether they're intelligent, etc. A fantasy that takes place in an enchanted wood might require you discover all the varied creatures who live there. Even an underground vampire nation that lives in Mexico might need this sort of development.

Regards,
Michelle