Meta descriptions live in the page's `<head>` as `<meta name='description' content='...'>`. Search engines typically display the first 150–160 characters in desktop results and slightly less on mobile.
Google does not use the meta description as a ranking factor, but it strongly influences click-through rate from the search results. A specific, compelling description against generic competitors can dramatically outperform a higher-ranking page with weak copy.
Google also frequently rewrites meta descriptions — sometimes pulling a relevant sentence from the body of the page instead. This usually happens when the meta description does not match the query intent. Writing descriptions that work for your highest-value queries is usually the right priority.