The HTML code describes to the browser how a page is structured, and which elements is included on it: headings, paragraphs, images, lists, links, forms, input fields, and so on. Almost every page on the web consists of HTML code, and every part of those pages is a HTML element. The block of text that you are reading, for instance, is a <p> element, used for text paragraphs. The title that says “HTML” up above is a <h1> element, used to indicate titles. The programming language CSS is used to define the styling (layout, typography, colors, etc.) of HTML elements.
In WordPress, you can use HTML code to format your posts and pages when you have the text editor active in the post editor.