It was an incredibly simple concept: write a plain text file which includes information and link the file to related files simply by including the related file names in <> brackets. Running the program with these files, the user could jump instantly from one file to the next. There was an associated editor, Transtext, which can make the same jumps and has very powerful macro capabilities. There was and still is nothing like this on the market in terms of this speed and simplicity - navigating thousands of linked files in Hygen on a 286 computer with 2 meg of RAM was not simply fast, it was instantaneous.It is true that the concept is simple, but any application that involves free links, like nvAlt or Notetab, for instance, reproduces it. So, the claim that there still is nothing like it is simply false. Most wikis support that idea. And even the simple-minded AhK scripts I recently published here do allow for such jump. You could even use brackets like "<" and ">", though I don't know why you would.
See also this earlier post.