An HTML summary that search engines may use for a result’s snippet. Its wording is not guaranteed to appear.
A meta description is a short summary supplied in a page’s HTML metadata. It gives a search engine a candidate description of the page. Unlike the main body text, it normally is not displayed as a paragraph within the page itself.
The useful editorial question is what someone needs to know before opening the page. A good description can identify the subject, the information available and a relevant distinction. It should match the real content rather than promise prices, availability or outcomes that the page does not support.
Treat the description as a summary you can write, not as a reserved advertising slot. Google may use it for the search snippet or choose page text that better answers a particular query. Saving a new description in an SEO plugin therefore does not prove that the live search result has changed or that more people will click.
Takeaway: Write accurate descriptions for important pages so prospective visitors can understand the destination. Prioritize useful specifics over a list of keywords or a phone number inserted into every page.
Example
A conference venue could summarize a meeting-room page by mentioning room layouts, capacity information and booking requirements, if those details are actually present. A description promising “instant booking at the lowest price” would be inappropriate if the page only offers an inquiry form and makes no such commitment.
What to check
There is no universal description length that guarantees full display. Search snippets can be shortened to fit available space. Use enough text to communicate the page’s value clearly, then check that the published metadata is present and consistent with the page.
For more detail, see Control your snippets in search results.