Header tags are HTML tags used to set apart headings and subheadings from the rest of the content on a webpage, in descending order of importance from H1 to H6. Most often, you’ll only need H1 through H4, where: H1 is the primary heading and title for the page H2 is a subheading and supporting point H3 is a subsection or list item under an H2

To summarise, H1 is the main heading or topic of your writeup. H2 to H5 are hierarchical in-content subheadings. In most scenarios, you only have to use H2s all through your content but if you have to further explain or breakdown your H2, then use H3. Use H4 to further explain or break down H3 and so on.