Just like the description on the back of a book cover, in SEO, a meta description is a very brief paragraph explaining what a search result is about. A good meta description needs to be short, convincing, relevant, and targeted.

Meta Descriptions In Action
Meta descriptions let users searching online see a brief summary of what a link is all about in order to help them decide whether or not to click and read the whole thing.
Crafting a great meta description will not only improve your SERP ranking, but will also help you get more clicks, thereby increasing your website traffic.