Markdown is a text formatting engine. When you are on the steemit editor, there is the word Markdown on the upper right. You can see the pictures below and these links for more:
1 - https://guides.github.com/features/mastering-markdown/

2 - https://steemit.com/steemit/@scrooger/markdown-editing-for-steemit-posts-easy-copy-and-paste