Markdown is just code you put into your post to format it.
You can do bold, italic
Headers
Of
Different
Sizes
- Unordered
- Lists
- Ordered
- lists
And lots more. Just click on the "Styling with Markdown is supported" link when you are entering your post on the steemit website. It's a good practice to use your word processor to write, then format on this site. Formatting makes your article easier to read,