This is just a basic "fun" tag and serves little practical purpose on it's own. Enclosing text in this allows the viewer to "edit" whatever text is enclosed. Don't worry, the change is not permanent! :-)
Very simple enclose the text in these tags:
<div contenteditable="true">Enclosed text here.</div>
Result: Anywhere on this body section you can delete or add text.
Told you it's not much use, but if you are bored or in a bad mood just download some page from the web that you hate, enclose it with this tag then delete until you feel better!