> What is the meaning of "hidden"? Maybe it's worth adding a legend.

Hidden = Not displayed on the https://www.scheme.org/ front page.

A legend is a good idea, and the appearance of the page could be
improved in general. Pull requests and patches welcome. The file to
change is lib/www.scm in https://github.com/schemeorg/schemeorg. I will
rest for a bit before writing more code for the site.