About the "hidden text", i assume you're talking about wikicode comments. If so, choose "Comment" in the "Insert" menu of the visual editor, or source-edit your page with a code like this:
<!-- A comment on one line -->
<!--
A comment
on many lines
-->
For those small images/icons in your other screenshot, it's possibly a template they are using, would need a link to the page where the schreenshot is taken from to tell.
But at the end, it's basically just something like