These are elements of the HTML server-side scripting language which are used as headers or as a means to give a header to some section of your content. These heading and subheading tags are critical for search engine marketing as if by any chance they happen to be in a graphical form, they essentially become unreadable to search engines. They must ideally be used as per relevance, such that the more prominent ones are enclosed within <h1> while the less important ones are enclosed within <h2> and so on.