A "Superdoc" is a document that has Everything's embedded perl functionality. Something that looks like this:
[%
"hello $$USER{title}!";
%]
Would print out "Hello nate!" on my screen... or whoever was browsing the site. These chunks of embeddible perl behave like perl functions -- usually they concatenate a string that is then returned and embedded in the document.
Superdocs are a great way to write CGI scripts, because it allows us to create nodes like Page of Cool or
Everything's Best Users... and I can code them up and debug them right in my browser.
Superdocs also have the ability to call chunks of htmlcode -- these are nodes which are reusable chunks of embeddable perl and can optionally take parameters:
[{textfield:title}]
would create a textfield that would let you edit the current node's title -- provided (of course) that you had the correct permissions.