HTML is a markup language—not a programming language—that uses tags to describe the structure of a webpage. These tags tell the browser how to display text, images, links, lists, forms, and other content.
In other words:
A semantic element tells you what kind of content it contains just by looking at the tag.
Examples of semantic elements:
< header> — top section of a page
< nav> — navigation menu
< main> — main content
< section> — a section of related content
< article> — a standalone piece of content
< footer> — bottom section of a page