A meta description is an HTML element with a one- or two-sentence summary of the page. Search engines often show it under the title in results, but may write their own snippet from the page instead.
Why it matters
It isn't a direct ranking factor, but it works like an ad for your page. A clear description that matches the search can raise your click-through rate.
Example
"Learn the right cold brew ratio for any batch size, with a printable chart and tips for stronger or smoother coffee." tells searchers exactly what they'll get.
What to do
Summarise the page honestly in about 150 to 160 characters, include the main topic naturally, and give every page its own description.