HTML stands for Hyper Text Markup Language. It's the code we use to display content on web pages.
It uses HTML tags to "markup" the content on a page.
HTML tags are keywords surrounded by angle brackets < >, and there is usually (but not always) an opening (or start) tag <keyword> and a closing (or end) tag </keyword>.
For example, the start tag for a paragraph is <p> and the end tag is </p>.
Go to www.example.com, right-click (or ctrl-click), and choose View Page Source to see the HTML behind the page. (Note: you can do this with any web page, and frequently will, to see how different features on pages are constructed.)