Explain what the `<header>` tag in HTML signifies and how it is used in the structure of a web page.

The <header> tag in HTML is used to define the top part of a section or page, which usually contains headings, logos, navigation links, and other introductory information. It helps structure the content, making it more semantically understandable for browsers and search engines.

Example of usage:

<header>
  <h1>Site Name</h1>
  <nav>
    <ul>
      <li><a href="#">Home</a></li>
      <li><a href="#">About Us</a></li>
      <li><a href="#">Contact</a></li>
    </ul>
  </nav>
</header>

Multiple <header> tags can be used on a page, for example, inside <article> or <section>, to denote the header part of a specific section.