The heading is simply like a title. In HTML, it is structured by using heading tags such as H1, H2, H3, and H4. Normally, as the heading number gets bigger, the font gets smaller.Tags are similar to, but more specific than, categories. The use of tags is completely optional.
WordPress.org Support
Example: A well placed heading can be used to change the separate topics in a post and structure my work.
For those of you who don’t know what a tag is. A HTML tag is:
An HTML tag is a code that describes how a Web page is formatted. HTML tags are defined by the characters < and >.
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About Headings
The headings that we use in WordPress are HTML headings that separate content on your posts. Headings are important because they let the writer of the posts change topics or shift the post directions. They also let search engines use the headings to find your posts and most importantly they structure the post you are writing, giving it a change in direction or introducing a new topic or idea.
Headings can be used in both the editor options. In the visual editor the Headings tool looks something like this:

In the text editor, the Headings look like this:

Search Optimization
Headings also allow search engines to find your content. They often times view your headings and select them if they match the searched ramifications. This will help bring more traffic to your site.
I had a great opportunity to interview, Lorelle VanFossen. Lorelle is a keynote, trainer, and writer, consultant on web writing, blogger, author, and teacher, plus offers a variety of training programs that emphasizes on web publishing and the social web. This gives her the opportunity to travel to places like Holland, Texas, and Canada to name a few to help inspire and motivate individuals to “have their say” on the web. As you have noticed, Lorelle is the Jill of all trades.