What is DOM element?

Document object model.
The DOM is the way Javascript sees its containing pages' data. It is an object that includes how the HTML/XHTML/XML is formatted, as well as the browser state.

A DOM element is something like a DIV, HTML, BODY element on a page. You can add classes to all of these using CSS, or interact with them using JS.

It's actually Document Object Model. HTML is used to build the DOM which is an in-memory representation of the page (while closely related to HTML, they are not exactly the same thing). Things like CSS and Javascript interact with the DOM.