Your pitch in the search results — the clickable title and the summary beneath it.
The title tag and meta description aren't on the page itself; they're how your page sells the click in search results. A perfectly structured page underperforms if its title is generic or truncated and its description gives no reason to click.
Write the title for the searcher, lead with what they're looking for, and keep the description accurate — overpromising earns clicks but also bounces, which undoes the gain.