<meta name="description" content="..."> lives in the page's <head>. Google sometimes uses it as the SERP snippet and sometimes rewrites the snippet from the page body — whichever it judges more useful for the query. Either way the meta description is the version *you* control, so it should answer the most likely query the page targets.
Length: 140–158 characters is the safe display window before Google truncates. Write one verb-led sentence that names the page's primary keyword and the user's intent. Stuffing keywords here is wasted effort: Google does not use the meta description as a ranking signal, only as a snippet candidate.
Every public page should have one. Missing meta descriptions force the SERP snippet to be auto-generated from page text, which often looks empty or repetitive.