What it does
It is written in the page head as <meta name="description" content="...">. Search engines may use it as the snippet under your title, or may write their own from the page if they think it matches the query better.
Writing a good one
- Say what the page offers and why it helps, in plain language.
- Keep it to about 155 characters so it is not cut off.
- Give each page its own, not the same text across the site.
Check the length and look with the SERP Preview, and see the guide.
Common misunderstanding
A meta description does not directly affect ranking. Its job is to persuade people to click. Related terms: SERP.