The imaginary world that you build could be this one in, say, a thousand years time or it could be set on a different planet with an environment we can only imagine. Its inhabitants might experience weightlessness, or have to survive in temperatures of minus 200 centigrade.
Part of the fun of creating your own new world is that you have to create both physical factors – such as gravity, temperature, or whatever – but also create a culture for its peoples. If your science fiction is well created, the environment will dictate culture which in turn will define character. So setting and characterisation become entwined.
All this, and more, is explained as both the skill and the enjoyment of writing for the genre in Lazette’s Writing Science Fiction. And you are even shown how to market your creation once you have written it.