A meta description is the short summary of a page that you put in the HTML for search engines. The text below a search result is called a snippet. Google writes in its explanation of snippets that it mostly creates snippets from the page content, and sometimes uses the meta description when it gives a more accurate description.
In the SEO Starter Guide, Google describes a good meta description as a concise summary of one or two sentences.
How long can it be?
Google sets no maximum, but truncates the snippet shown where needed, usually based on screen width. Put the most important information first.
Example
<meta name="description" content="Made-to-measure dormer windows in Apeldoorn and the surrounding area. Measuring, permit check and fitting by one regular team. Request a quote.">Common mistake
Putting the same or nearly the same description on every page. Google writes that this does not help. Give every important page its own description that matches what is on that page.
Worth knowing
If you do not want certain text in a snippet, use data-nosnippet on that part of the page. With nosnippet, Google shows no snippet for the whole page. Google writes that a page must be allowed to show a text snippet to be eligible for AI Overviews and AI Mode.
See also title tag and click-through rate (CTR).