Not sure what lesson you are, so please post a link.
A document type declaration is the signal given to the browser that tells what namespace to look up for supporting code; and, the name of the root element. This is the doctype for HTML5...
When there is no version or namespace information included, the browser defaults to HTML5. The root element of the document is
<html></html>. There will only be one, and all document markup will be enclosed between these two tags.