Heading tags are used to define the headings in your page. Heading tags will be recognized by browsers which don't recognize style sheets. Heading tags are used by search engines to identify words which are more important than the rest of the page text. The theory is that headings will sum up the topic of the page, so they are counted as important keywords.
Heading tags are part of the HTML coding for a Web page and relate to the visible text on the page and help tell search engines what that page is about. Headings are defined with H1 to H6 tags.