What are heading tags?
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. Search engines consider Heading text more important than other text on a web page.

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
H4 is a subsection or list item under an H3
Etc.

Why are header tags important for SEO?
Heading tags outline the topic of your main article or a section within the article. Search engines and users read the heading tags to understand the article's content without reading the entire article. For example, if my H1 heading would be "How to optimize your website for search engines" you would expect the rest of the article to speak about the SEO process and not something else.