What is Markdown? A friendly reminder from Wikipedia: Markdown is a lightweight markup language with plain text formatting syntax.
What this means to you is that by using just a few extra symbols in your text, Markdown helps you create a document with an explicit structure. When you take notes in plain text (in a notepad application, for example), there's nothing to indicate which text is meant to be bold or italic. In ordinary text, you might write a link as http://example.com one time, then as just example.com, and later go to the website (example.com). There's no internal consistency.
But if you write the way Markdown prescribes, your text has internal consistency. Computers like consistency because it enables them to follow strict instructions without worrying about exceptions. So if you know that ** on either side of a phase will bold it, you can use that across various text editors. A text file saved with these symbols will read consistently across different editors and viewers. Many social media sites will allow these symbols (but not tell you) so try them out there too. Some editors offer a menu bar with bold, italics etc but they are just inserting these symbols into the text for you.
For a long time, I thought all the files I saw on GitLab and GitHub with an .md extension were written in a file type exclusively for developers. That changed a few weeks ago when I started using Markdown. It quickly became the most important tool in my daily work. Markdown makes my life easier. I just need to add a few symbols to what I'm already writing and, with the help of a browser extension or an open-source program, I can transform my text into a variety of commonly used formats such as ODT, email (more on that later), PDF, and EPUB.