Final answer:
The DOCTYPE tag assists the web browser in correctly displaying a web page by specifying the version of HTML to be used for rendering.
Step-by-step explanation:
The DOCTYPE tag is a declaration that helps a web browser to display the web page correctly. The declaration is not visible to users but is used by the browser to know what version of HTML the page should be rendered in. It must be the first line in an HTML document, before the tag. The DOCTYPE declaration is essential for browser compatibility and to ensure that the page adheres to the HTML standards set by the World Wide Web Consortium (W3C).