The doctype declaration is not an HTML tag; it is an instruction to the web browser about what version of the markup language the page is written in.
The doctype declaration refers to a Document Type Definition (DTD). The DTD specifies the rules for the markup language, so that the browsers can render the content correctly.
The "transitional" qualifier means that the code can use deprecated (obsolete) structures and elements, and the browser, if it supports them, should render them.
The DTD is recommended for every web site.