The information that is displayed by the search engines in the SERPs about a particular page or website. Your meta descriptions should be no longer than 156 characters. Anything more will get truncated by Google. Your pages should all have meta tags, none missing. Your pages should all have unique meta tags, no duplicates.
Why do Meta tags matter?
They matter because they act as descriptors for your site. They tell the user what content they can expect to find when they click on the result. While they have no direct ranking impact, they increase SERP click through rate, which is a ranking factor.