A Meta Description is the brief description that appears below your blue link heading in a SERP. Typically, this is under 160 characters long and should describe the contents of the page in order to help users understand what the page is about.
A well-written Meta Description encourages users to click through, provides more likelihood of your page being clicked over others, and increases social media visitors. Therefore, even though your Meta Description does not directly impact SEO, in writing an effective summary of your page you are able to indirectly improve your ranking & visibility.