Headings are the titles of a page and its sections, ranked from 1 to 6. The h1 is the main title of the page, h2s open the main sections and h3s the parts within them. According to the W3C explanation, browsers and assistive software such as screen readers use headings to navigate within a page. Someone using a screen reader can jump from heading to heading.
Why does it matter?
Headings show what a page is about and where an answer is. According to its explanation of title links, Google uses the main heading as one of the sources for the title in search results. Google does write in the SEO Starter Guide that the order of headings makes no difference to Google. For accessibility, the W3C recommends not skipping levels, so no h4 directly after an h2.
Example
<h1>Having a dormer window fitted</h1>
<h2>What does a dormer window cost?</h2>
<h2>Do you need planning permission?</h2>
<h3>Dormer at the back of the house</h3>
<h3>Dormer at the front of the house</h3>
<h2>How does the fitting work?</h2>Common mistake
Choosing a heading level for its font size. The structure then says nothing about the content. Handle styling with CSS and use headings only for section titles. Write headings that say something on their own as well: “What does a dormer window cost?” is clearer than “Prices”.
More in Writing content that AI assistants can cite and under title tag.