HTML Heading 1, often referred to as H1, is the most important heading tag in HTML (Hypertext Markup Language). It is used to define the main title of a webpage, helping both users and search engines understand the primary topic of the content. An H1 tag typically appears at the top of a page and should be unique to each page to enhance search engine optimization (SEO). In HTML, the H1 tag is written as `<h1>Your Title Here</h1>`. It is crucial for web accessibility, as screen readers use it to navigate the content. Proper use of the H1 tag contributes to a well-structured document, making it easier for readers to find relevant information quickly.