A meta description will not rank you on its own, and people use that as an excuse to phone it in. Wrong move. It is the sentence that decides whether your ranking turns into an actual visit.
The short version
A meta description is the short HTML snippet that summarises a page in search results, usually shown under the title. It is not a direct ranking factor, but it strongly affects whether people click.
How to write one that works
Keep it around 150 to 160 characters
Lead with the benefit or the answer
Include the keyword naturally, engines bold matches
Treat it as ad copy, give a reason to click
One catch
Google sometimes rewrites your description using text from the page. Write a strong one anyway, because when it does get used, it is doing the selling for you.
FAQ
Does a meta description affect rankings?
Not directly. But by lifting click through rate, a good one can indirectly support performance and is always worth writing well.