Marking up a document involves wrapping text with tags. The above is a DIVision tag for sectioning the page. In the pre-HTML5 days, we used
<div></div> with special id attributes to section a page into HEADER, CONTENT, FOOTER, but we now have special elements for that purpose.
<div id="header"> </div> <div id="content"> </div> <div id="footer"> </div>
Sectioning helps us to organize our document by grouping related elements in a container for easy identification. Markinp up a document is akin to writing an outline of the content.