Meta descriptions provide a short explanation of a page’s content. Search engines don’t use meta descriptions themselves as a ranking factor, but they do use them in combination with the title tag and URL of a page create its search snippet. Search snippets are the title and description displayed in search engine results. Search engines will scrape page content it thinks is relevant to a search for pages that have missing or inaccurate descriptions.
Here’s the search snippet in action for Google, Bing and Yahoo!, respectively: