It's an editor for those JSON file formats which shows contextual help as you edit the files, similar to an IDE. It works by parsing the javascript and figuring out where your cursor is relative to the syntax of that particular format.
Likewise. The landing page gave me no clue. I just now went back and clicked on one of those documents, assuming that I'd get some kind of documentation about the format. Turns out it opened a live document editor. I certainly wouldn't have predicted that from the line "Open-source documentation for common JSON formats."
It's documenting the structure of json files used in large projects. On of the three, for example, is documenting the package.json file used by npm. It gives an inline editor to view what one looks like.