Heading tags, as their name suggests, are used to differentiate the heading of a page from the rest of the content. These tags are also known to webmasters as HTML header tags, head tags, heading tags and SEO header tags. The most important heading tag is the h1 tag and least important is the h6 tag. In HTML coding the header tags from h1 to h6 form a hierarchy. This means that if you skip any of the tag numbers (ie. jump from 1 to 3) the heading structure will be broken, and this is not ideal for on-page SEO.

Eg:-if your site is introduced with a heading in h1, and a sub-heading in h3, the hierarchy will be broken, meaning the heading structure is not SEO-friendly. The coding should be something like what is shown below:

<h1>Main Heading</h1>
<h2>Secondary Heading 1</h2>
<h3>Sub-section of the secondary heading 1</h3>
<h2>Secondary Heading 2</h2>
<h3>Sub-section of the secondary heading 2</h3>

Advantages of Using Heading Tags:

The heading tag is used to represent different sections of web page content. It has an impact on both the SEO and usability of your site.

Header tags from an SEO point of view:

Relevancy: Search engine spiders check the relevancy of the header tag with the content associated with it.
Keyword Consistency: The search engine spiders check the keyword consistency between the header tags and other parts of the page.
The Importance of an h1 Tag: The h1 is the most important tag and it should never be skipped on a page. Search spiders pay attention to the words used in the h1 tag as it should contain a basic description of the page content, just as the page <title> does .
Enriched User Experience: Heading tags give the user a clear idea of what the page content is about. Search engines give much importance to user-experience on a site, meaning the presence of heading tags becomes an important component of SEO.

Header tags from a usability point of view:

For users of the web who must use a screen reader, it is easier to navigate sections of content by referring to properly structured headings on a page.
The h1 heading tag (main heading) of a page gives users a quick overview of the content that is to follow on the page.
By reading the different heading tags, users can scan a page and read only the section they are interested in.
The primary use of heading tags is for SEO, not to gain the larger, more prominent fonts; but the presentation of a web page does look cleaner with the presence of these tags.

Things you should not be doing with heading tags:

Do not stuff your heading tags with keywords.
Do not use more than one h1 tag on a page unless really necessary. Usually pages will have a single h1 heading and including two might make search engines think this as an attempt to include more keywords for SEO in multiple h1 tags. It is better to divide the content into two separate topics on individual pages with their own h1 tags. This makes more sense to both readers and the search engine spiders, however, using multiple h1 tags is allowed.
Do not use heading tags as hidden text. Any hidden text can result in penalties for your site, especially if the hidden part is a component that effects SEO.
Do not repeat heading tags on different pages of your site. It is a good practice to have unique heading tags throughout your site.
Do not use the same content in your page’s h1 tag as in your meta title tag.
Do not use heading tags for styling text but use them for presenting organized and structured content on pages. Use CSS stylesheets for the purpose of styling.