It stands for "Hyper Text Markup Language".
It is a programming language that is used to create web sites. It tells the computer what images and text to display, as well as where on the screen to display it.
You can view the code that was used to make the web site you are viewing, simply buy going to the view menu (on top) and clicking view source or page source. Getting this might be a little different depending on your Internet Browser.
HTML is a standard, meaning every web browser should know how to interpret it because it has certain rules. This rules are dictated by the World Wide Web Consortium an international group who make web programming standards.