What a meta description is
The meta description is <meta name="description" content="..."> in the page head. It is a short summary that search engines may use as the snippet below the blue title link. Social platforms also use it when no Open Graph description is set.
It is not a ranking factor, but it is your sales pitch on the results page, so a clear description can raise click-through rate.
Ideal length
Google truncates by pixel width, not characters. Around 150 to 160 characters usually fits on desktop and roughly 120 on mobile, so put the key point and keyword early. A description far shorter than that wastes space; a longer one is cut off with an ellipsis.
Why Google rewrites it, and how to write a good one
Google writes its own snippet from the page text when it thinks that matches the query better, which happens for most queries. You cannot prevent it, but a specific, accurate description is used more often.
Write a unique description for every important page; summarise what the reader gets and why it is useful; include the main keyword naturally (Google bolds matching words); add concrete details such as prices, numbers or features; and avoid duplicates, which Search Console and audits flag. Pair it with a strong title tag.
Example
A meta description:
<meta name="description" content="Track daily Google rankings for 100 keywords, see the pages you compete with and get an AI action plan. Start for $5, no subscription.">