It's like story writing. but the part that comes before the story. when you come up with the world in which the story takes place.

Like when JRR Tolkien wrote LoTR, he first had to build the world of Middle Earth. He came up with the races that inhabit the world, the geography, the politics, the languages everything that sets the world of Middle Earth apart as a fictional setting for the story of LoTR to take place in.

Every good Fantasy or SciFi story first has to have a world established that it can take place in.

And there are a lot of people who enjoy the process of coming up with these worlds with or without the story to be written in it.

For instance, I have a world I've been thinking about lately that's set in our distant future where humans have built a massive spaceship to evacuate the earth before its destruction in search of another inhabitable planet. The worldbuilding I'm doing involves designing the ship, theorizing how their government structure works as they're flying through space for thousands of years, and deciding on some of the culture and language that might develop as they drift further from their roots and get used to life in essentially a small manmade planet. I don't really know if it's viable, but I thought it was interesting idea.

One of my favorite worldbuilding projects on r/worldbuilding is a society of humanoids who live in a continent-sized cave system. There's magic and elves (but not the same as your typical elves), humans, of course, but they're not the dominant species.