A meta description tag is a snippet of HTML code in a web page header that summarizes the content that is on the web page. The meta description is usually placed after the title tag and before the meta keywords tag. When optimizing a web page for search engines (SEO), it is considered a best practice to use meta description tags.
Before Google and other search engines changed how much importance they place on meta description tags, the keywords in a meta description could influence ranking on a search engine results page (SERP). Google announced in September of 2009 that neither meta descriptions nor meta keywords factor into Google's ranking algorithms for web search; however, the meta description may appear on the search engine results page if Google does not find a snippet on the page that contains the keyword the user is searching for.
See also: keyword stuffing, meta tag
Read more about meta description tags:
Learn more in a beginner's guide to SEO
Get best practices on creating unique meta descriptions
Read Google's advice on meta tags