What it is
Four tags do most of the work: <title>, <meta name="description">, og:title and <link rel="canonical">.
The meta description in particular is short, declarative and easy to find, so it is often what a search result or a summary shows about you.
Why an agent cares
If you don't write that sentence, a model writes it for you from whatever copy it found first, and you don't get to review the result.
A missing canonical means the same content under several URLs gets treated as several pages, splitting whatever authority you have.
How to fix it
Write the description as a statement of fact, not a tagline. "Acme is a payroll platform for UK companies under 50 people" beats "Payroll, reimagined."
- One
<title>per page, specific to that page. - A description that names the category and the audience.
- A self-referencing canonical on every page.
How we test it
We look for title, description (or og:description), og:title and canonical. Three of four passes.