That is, the <hr> element. With the help of a few contributors, I put together this page of very simple styles for them. You could get a lot fancier with an element like a <div> that can hold content, but I like the semantics of a horizontal rule. It’s an emphatic break between two sections of content.
Note that in some of these examples, generated content is used (:before/:after). This isn’t supposed to work, as far as I know, because <hr>s are no-content style elements (they have no closing tag). But as you can see, it does. If you are concerned about uber-long-term design fidelity, stay away from those.