Things which are deeply intertwingled. Joe Armstrong (Erlang) gave at least one talk about it, available on YouTube:
https://www.youtube.com/watch?v=Uv1UfLPK7_QThe key points were that Wikipedia style wikis force you into a page-at-a-time model. And TiddlyWiki uses more like a post-it-note sized amount of information. Wikipedia has a few links, TiddlyWiki items would have dense links to other notes. Then the experience of reading something would be like a 'choose your own adventure' book - click on a link and you don't get whisked away to a new page with all the context lost, you get the next note appearing below. It's building on the work of Ted Nelson and the early HyperText dreams from before the Web.
As you click and explore, you build up a longer page of just the ideas and content you want, and don't have to write huge topic-wide essays and browse huge topic-wide pages all in one go. He called it "the correct level of granularity, deeply intertwingled".
There were probably other important points I've forgotten.
Relevantly for Dendron, the slide I've clicked onto in the YouTube has a quote: "Hierarchical and sequential structures are usually forced and artificial. People keep pretending they can make things hierarchical, categorizable and sequential when they can't" - Ted Nelson, 1974.