Worldbuilding is basically creating a world, complete with culture, language, geography, and sometimes even stories that take place in that world
The fascinating thing about worldbuilding is that one has to take into consideration at least most of the things that make a real world tick. A useful side effect is that one gets tons of useful knowledge (I learned a lot about biology and DNA because I wanted to make a scientifically sound species in my world). Of course history is the most useful subject because then there can be fascinating wars and political goings-on in the world too.
There are many good resources for learning how to worldbuild, but the best place to learn is from sci-fi and fantasy authors. The best I have found are the Dune series, the Crown of Stars series by Kate Elliot, and the Ender’s Game books.
Besides being useful, worldbuilding is a damn good way to waste time as well, and a great way to zone out.