The `<h1>` tag is an HTML element used to define the most important heading on a webpage. It typically represents the main title of the content and helps search engines understand the topic of the page. Each webpage should ideally have only one `<h1>` tag to maintain clarity and improve SEO. Using the `<h1>` tag correctly enhances user experience by making it easier for visitors to identify the primary subject of the page. It is often styled with larger font sizes and bold text, making it stand out. Proper use of headings, including `<h1>`, contributes to better organization and accessibility of web content.