A comment is a piece of text that is not seen on the rendered page.
It's an ancillary note about the content of your page.
The syntax follows the markdown syntax (ie XML / HTML)
<!-- This is a comment and will not be visible in the rendered page -->
By default, the comments are stripped off and are not part of the created page. If you want to have the comments (as comment ie still not visible) in your page, you can set this configuration to on