Your page has several audiences that never render it the way a person does. Search engine crawlers read the <title>, the description and the canonical URL to decide what to index and how to present it. Social networks and chat apps fetch the page when someone pastes a link and build a preview card from Open Graph tags. Search engines and AI assistants read JSON-LD structured data to learn that a page is a product with a price or an article with an author. All of this lives in markup you write once, mostly inside <head> (Head).
This section covers the HTML side. Search Engine Optimization explains how search engines use these signals for ranking, rich results and AI answers.