Aside from the title tag, which isn’t really a part of the visual page, the heading tags can provide both visitors and the search engines significant cues to the importance of your on-page content. There are six different heading tags: H1, H2, H3, H4, H5, and H6. Collectively these are referred to as Hx tags.
The value of the Hx tag is much the same as the value of your main points in an outline for a book or paper. Your outline shows your main topics and sub-topics. These are often then used as section headings in your final draft. Your h1, for example, would be the page topic. Your H2s would then be your main points (I. II. and III.) Your H3s would be your first sub-points (A. B. C.) and your H4s would be your second sub-points (1. 2. 3.) etc.