Comments are useful detailed description for readers to understand the line of markup code. These will be ignored by processors.

Markdown has no inbuilt in support for comment syntax, but extended syntax vendors like GitHub and stackoverflow has provided the syntax, and can not be used with markdown syntax.

Html comments can be used in markdown code, <!--- Wrap text ---> is the symbol which will be wrapped with comments text. Some markdown processors supports two dashes Html Syntax <!-- Wrap text -->

here is markdown comments syntax

This comment syntax will not work when parsing using pandoc markdown , comments shown in html.

_italic text_
comments syntax
***  bold text**

and generated HTML code is

<p><em>italic text</em></p>
<!-- raw HTML omitted -->
<!-- raw HTML omitted -->
<p>***  bold text**</p>

and output is

italic text

*** bold text**

As you can see comments are not visible to user on browser