Open a message. Then click the More Actions pull-down menu at the top of the window, and choose View Headers or View Source.
Headers are where information about an email is stored:
who it's from and to, when it was sent, the subject, and
much more that is usually hidden from view.
The information shown will be quite detailed. This includes
the information shown in basic headers, plus other, advanced,
information useful in tracking where a message came from.
HTML source refers to the "code" behind what you see in a Web browser. This code contains all the information that a browser needs to show you a page with colors, pretty layout, and formatting (like bold or italics.)
When you view the HTML source of an email, you'll see the full headers, plus all the other code that the browser reads.