A meta description is the small blurb that appears underneath your website on the SERP.
It is designed to provide users with a brief summary of the content on your page so that users know if the page will answer their question. Traditionally, meta description length has maxed out at 155 characters, with the exception of experimental periods such as when Google temporarily extended the length to 320-character meta descriptions.
Meta descriptions have a strong impact on ranking, but this impact is indirect. There is no direct ranking benefit from meta descriptions. Google does not use meta descriptions as a factor in their ranking algorithm, and for other search engines it is not a key factor. This means that, as well as looking unattractive, ‘keyword stuffing’ (whereby websites pack their text with frequently searched words) does not work in meta descriptions.
So how do meta descriptions impact ranking? Well, Google uses click-through-rate (CTR) as a ranking factor, and may move a result up if more users click-through than would be expected from the current ranking position. Meta descriptions can persuade users to click on a page, so when they are strong and compelling they can drive up your ranking.