A meta description is the short summary of a page that search engines may show beneath its title in results. It lives in the page's code, not on the visible page, and is written for the person deciding which result to click. A good one says what the page offers and why to choose it, in a sentence or two.
It is not a ranking factor. Google also rewrites it often, pulling a line from the page instead when it thinks that line answers the search better. Writing a good one still raises your chance of being shown as you intended.
<meta name="description" content="…"> in the <head>. Google truncates around 155 to 160 characters on desktop and less on mobile, and substitutes page text for a large share of queries. One unique description per indexable page; duplicated or missing descriptions are flagged in most crawlers and lead to Google choosing for you.
Treat it as ad copy for the result: the primary phrase near the front so it gets bolded when it matches the query, a concrete benefit or differentiator and a reason to click. CMS SEO plugins expose the field per page and can template it for products and posts, which is better than nothing but worse than writing the important pages by hand.
<meta name="description" content="Emergency plumber in Riverside, at your door within the hour, day or night. Fixed call-out fee quoted before we start.">