How do you build the world you already live in?

The first time I met the term "World-building" I wondered what it meant. I play lots of board games with my sons, so perhaps world-building meant the art of setting up rules so a game wouldn't be too easily lost or won. I watched my sons play lots of computer games and wondered if world-building was the writing of long lines of computer code to model the buildings, hills and roads. I read a lot of science fiction too. Is world-building the task of adding science to imagination so a new world...