It's this: <hr>
It stands for horizontal rule and it does just that. It draws a horizontal line across your page. And with that line you create a footer, a space 'below the line' where you can add comments, links and information that you want to share with your readers, but that doesn't fit naturally in the main part of the post.
It's one of the most useful things I've learned in the last six months, and it's changed the way that I write my business blog.