HTML uses the <h1>, <h2>, <h3>, <h4>, <h5>, and <h6> tags to mark up content that can be classified as a heading. The "h" in these tags stands for "heading," and the number prescribes the level of importance, or hierarchy.

Think of these tags like headings in a school report. We usually make the main heading of the report the largest. Then sub-headings smaller. Sub-headings of sub-headings are usually smaller yet. We organize our content with a hierarchy defined by the size of the heading. This is the same way we should organize the content on a website.

The <h1> tag has the highest importance, and should usually contain the main heading for the page. The <h6> tag has the lowest importance. It is good practice to break your content up using headings, but it is not required that every page has <h1> through <h6> tags. The number of headings used should be determined by the content/sections.

Headings tags are the only HTML tags that use numbers to determine importance. It is common for beginners to add numbers to other tags such as paragraphs (ex. <p1>, <p2>, <p3>, etc). This is not correct, and should be noted so you don't make this mistake.