Headings serve as the waypoints that outline the content of your web page. Proper headings signal search engines about the available content and, more importantly, provide a navigational framework for assistive technologies. Most sighted users skim web pages for content rather than reading every word, and headings enable this ease of navigation for everyone.
The structure of headings is hierarchical: H1 represents the top-level navigation, H2 labels related topics, and H3 through H6 are used for subtopics. Heading tags can be repeated with a topic change but must always follow a sequential order. For instance, you cannot jump from an H1 heading to an H4 without including H2 and H3 headings in between.
Headings should not be used to change the text size on web pages; they must be used solely for the logical organization of content.