Markdown lets you compose posts and comments containing links, lists, and other styles using regular characters and punctuation marks, instead of complicated HTML code.
Markdown is a simple formatting language for writers that enables you to you easily create headers, lists, links and other HTML when creating or editing content or adding comments on the front end. With this feature enabled, you can write Markup directly in the editor and see it displayed as HTML on the front end.
Likewise, when someone posts a comment, they can also use Markdown.
To enable this feature, go to: Jetpack Settings > Writing and enable Markdown, optionally also enabling it for comments.
If you find yourself using Markdown regularly, you may enjoy this feature from Jetpack.