Headings are used to highlight titles in sections of content for your site. In the drop down from the tool bar above the page or post that is being edited, you'll see options for Paragraph (which is default, Heading 1, Heading 2, Heading 3, Heading 4, Heading 5, and Heading 6. You are able to highlight any text within your post or page and choose one of these heading options.
Headings are primarily used for SEO purposes, and not necessarily for aesthetic reasons. See this article from Yoast that explains how to properly use them.