is a form of content editing that is used by many serious content producers. It is often preferred over rich text editors as it focuses users on the content - rather than its formatting. It provides basic 'markup' - capability to describe the document. Things like what are headings, what is bold, links - even images and tables.
However it limits your ability to change layouts and styling. We often want you to leave the formatting and styling up to us - and instead focus on the content itself.
This helps to ensure the website is consistent in its styling, spacing and layout. It makes the website look professional and tidy.
It also allows us to control the layout when it comes to displaying this content over a myriad of different devices, with different screen sizes. If you dictate the layout in a rich text editor, this may look good on your screen, but probably won't on lots of other devices. This is a critical part of having a professional modern website.
Markdown is simple to learn - it uses basic symbols to indicate what lines are headings, dashes or numbers for lists and so on. A comprehensive 'cheat sheet' help document
that can be found here