The meta description is a short summary of a page, placed in the <head>. Search engines often use it as the grey snippet of text under the title in results.
<head>
<title>How to Choose a Laptop for Students | TechShop</title>
<meta name="description" content="A practical guide to picking a student laptop — what specifications actually matter, what to skip, and how much you need to spend.">
</head>Google has stated publicly that meta descriptions are not used as a ranking signal. Writing a keyword-packed description will not move a page up.
That does not make it pointless. The description is advertising copy sitting directly under your headline at the moment someone is choosing between results. It affects whether people click, and clicks are the whole point of ranking in the first place.
This is a genuinely useful distinction to hold onto: some things affect where you appear, and other things affect whether anyone chooses you once you appear. Meta descriptions are entirely the second kind.
If a search engine judges that a different piece of text from the page answers a particular query better, it will show that instead of your description. This happens often, and it is normal — the same page may show different snippets for different searches, because different parts of it are relevant to each.
Writing a good description is still worth doing: it is what gets used when nothing on the page fits better, which is most of the time for your main target query.
For a large site with thousands of similar pages — product listings, for instance — hand-writing a unique description for every page is often not a good use of time. A search engine generating a snippet from the page content is usually better than a thin templated description repeated across hundreds of pages.
Write them by hand where it matters: the homepage, key landing pages, and any page you are actively trying to rank.