The header tag in HTML5 is a semantic element used to define a container for introductory or navigational content for a section or the entire page.

The h1 tag is used to define the main heading of a webpage or a section. It represents the most important heading and is usually placed at the top of the content hierarchy.

The h1 tag is often placed inside a header element to represent the main heading of the section or page the header belongs to. Placing h1 within header enhances search engine optimization by clearly structuring content.