HTML is the basic structure used to tell a browser what parts a webpage contains. It is like the frame of a house: it gives places for headings, paragraphs, pictures, and links.
When you read a page, the browser has used HTML to understand which words are a title and which are a button or a list. Other tools can change how those parts look.
HTML describes the content and its meaning, not the full visual style. Clear structure also helps assistive technology explain the page to people who use it differently.