It's just note-taking in a single app, but instead of organizing your notes into folders, you link them. It's what Obsidian made popular. The name, because I use it for all facets of life, if I go to the doctor or dentist, I write it down, if I write an idea, or journal, it's all there. Tiago Forte coined the term initially. You find a note on my second brain about it, in case you want to know more (I also have related blog posts)

Ah kk, makes sense. I'm doing something similar with an app I'm building! Very interesting stuff!

Never knew it as that term though. You learn something new everyday.