Elements like header, nav, main and article that describe what content is, not just how it looks.
Semantic elements communicate the role of their content: header and footer for introductory and closing material, nav for major navigation, main for the page's primary content, article for self-contained pieces, section for thematic groupings, aside for tangential content, and figure with figcaption for annotated media. They replace generic divs with names that browsers, search engines and assistive technologies can actually understand.
The benefit is practical rather than cosmetic: screen reader users can jump between landmarks like main and nav, search engines better understand page structure, and the code is easier for other developers to read. A classic question is when to use section versus article versus div. A good rule of thumb is that article stands on its own if syndicated, section needs a heading and groups related content, and div is the fallback when no semantic meaning applies. A page should also have only one visible main element.
What you'll walk away knowing