Actually H1, H2, H3 and so on are main part of your content and On page SEO. But talking about content element - it is just a title, where you can focus the readers attention better.
In the code they look like <h1></h1>, <h2></h2>. Depending on your site styles, you can have even H12. Well, in most cases H6 or H7 reach all the necessary limits.