A website can be seen as a book where, for example the sitemap is the table of contents. A book consists of chapters (within a website these are pages/categories) and the chapters contain paragraphs and sub paragraphs. For an optimal structure a page is using heading tags. In the source code, heading tags are visible with the code h1 till h6. It is important that the heading tags are used in the right way: if an h3 is used, the h1 and h2 should also be used.
This heading tag checker tool will show you the heading tags (h1 till h6) of a specific page. The page title will be shown as well. Make sure to use unique and relevant heading tags on every page.